Add arrows on the right of a matrix

Question

What's your question?

Currently I have this matrix:

Now I want to do a sum on this matrix by adding an arrow on the right which describes what I'm doing, like that:

What have you tried yet?

Well I found the witharrows package but I don't understand how I should apply it.

Minimal code

Code from first image

\documentclass[a4paper, 12pt]{scrartcl}

% =============
% Packages 
% =============
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}                % west european chars
\usepackage{amsmath, amsthm, amssymb}   % math symbols
\usepackage{blkarray}

\parskip5pt
\parindent0pt

% ------------
% Content 
% ------------
\begin{document}
\begin{align*}
    \begin{blockarray}{ccccccc}
        \begin{block}{(ccc|ccc)c}
            1 & 5 & 4 & 1 & 0 & 0 & | \cdot 2\\
            3 & 5 & 6 & 0 & 1 & 0 & \\
            1 & 2 & 4 & 0 & 0 & 1 &
        \end{block}
    \end{blockarray}
\end{align*}
\end{document}

Code from second image

So that's how I tried to apply witharrows to the matrix:

\documentclass[a4paper, 12pt]{scrartcl}

% =============
% Packages 
% =============
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}                % west european chars
\usepackage{amsmath, amsthm, amssymb}   % math symbols
\usepackage{blkarray}
\usepackage{witharrows}

\parskip5pt
\parindent0pt

% ------------
% Content 
% ------------
\begin{document}
\begin{align*}
    \begin{blockarray}{ccccccc}
        \begin{block}{(ccc|ccc)c}
            \begin{WithArrows}[c]
                1 & 5 & 4 & 1 & 0 & 0 & | \cdot 2 \Arrow{test}\\
                3 & 5 & 6 & 0 & 1 & 0 & 
                1 & 2 & 4 & 0 & 0 & 1 &\\
            \end{WithArrows}
        \end{block}
    \end{blockarray}
\end{align*}
\end{document}

but I'm getting a bunch of errors.

EDIT

Here are the error messages:

Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: You should use the command \Arrow only in the last of your environment {WithArrows}. However you can go on for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: The environments of witharrows ({WithArrows} and should not end by \\. However, you can go on for this time. No similar error will be raised in this document.
Document.tex|30 error| Misplaced \noalign.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: You should use the command \Arrow only in the last of your environment {WithArrows}. However you can go on for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Package witharrows Error: Your environment {WithArrows} has 2 columns and you try to use one more. Maybe you have forgotten a \\. If you really want to use more you should use the option 'more-columns' at a global level or for an environment. However, you can go one for this time.
Document.tex|30 error| Misplaced \noalign.

Answer 1

Probably it would be better if you used the nicematrix package (see below), but you can also add the arrow with TikZ and the tikzmark library.

\documentclass[a4paper, 12pt]{scrartcl}

% =============
% Packages 
% =============
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}                % west european chars
\usepackage{amsmath, amsthm, amssymb}   % math symbols
\usepackage{blkarray}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\parskip5pt
\parindent0pt

% ------------
% Content 
% ------------
\begin{document}
\begin{align*}
    \begin{blockarray}{ccccccc}
        \begin{block}{(ccc|ccc)c}
            1 & 5 & 4 & 1 & 0 & 0 & \tikzmarknode{times2}{| \cdot 2}\\
            3 & 5 & 6 & 0 & 1 & 0 & \\
            1 & 2 & 4 & 0 & 0 & \tikzmarknode{1}{1} &\\
        \end{block}
    \end{blockarray}
    \begin{tikzpicture}[overlay,remember picture]
     \draw[cyan,semithick,-latex] (times2.east) -- ++ (1em,0) 
      |- node[pos=0.25,right]{$(+)$} (times2.west|-1);
    \end{tikzpicture}
\end{align*}
\end{document}

With nicematrix it works very similar. Note that this code requires the latest (as of April 2021) version of the package.

\documentclass[a4paper, 12pt]{scrartcl}
\usepackage{tikz}
\usepackage{nicematrix}
\begin{document}
\[
 \begin{pNiceArray}{ccc|ccc}[last-col]
            1 & 5 & 4 & 1 & 0 & 0 & | \cdot 2\\
            3 & 5 & 6 & 0 & 1 & 0 & \\
            1 & 2 & 4 & 0 & 0 & 1 &\\
 \CodeAfter
  \begin{tikzpicture}
   \draw[cyan,semithick,-latex] (1-7.east) -- ++ (1em,0) 
      |- node[pos=0.25,right]{$(+)$} (1-7.west|-3-6);           
  \end{tikzpicture}
 \end{pNiceArray}
\]
\end{document}

Answer 2

Ashort code with pstricks`: I define two nodes, and connect them with the relevant node connection.

\documentclass[svgnames]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\usepackage{blkarray, bigstrut}
\usepackage{pst-node}

 \begin{document}%

\begin{align*}
& \begin{blockarray}{(ccc|ccc)l}
        1 & 5 & 4 & 1 & 0 & 0\bigstrut[t] & \rnode{A}{| \cdot 2}\\
        3 & 5 & 6 & 0 & 1 & 0 & \\
        1 & 2 & 4 & 0 & 0 & 1 & \pnode[0,0.5ex]{B}
\end{blockarray}
\ncbar[linecolor=Cyan, linejoin=1, arrowinset =0, nodesepA=2pt]{->}{A}{B}\naput{\color{Cyan}({+})}
\end{align*}

\end{document}