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.
3 & 5 & 6 & 0 & 1 & 0 &
, you've forgotten to add `\\` to start the new row.