Aligning multiline equations

Question

When using the multiline environment with the following code

    1. \(E(Y ) = aE(X) + b\). (5 marks)\par
\begin{multiline}
\begin{equation}
\E(aX+b)&=& \sum_{x}{ax+b)p(x)\\
\sum_{X}(ax\cdot p(x)+b\cdot p(x))\\
\sum_{X}ax\cdot p(x) + \sum_{X}b\cdot p(x)\\
a\underbrace{\left(\sum_{X}X\cdot p(x)\right)}_{E(X)} + b\underbrace{\left(\sum_{X}p(x)\right)}_{1}\\
a\cdot \E(X) + b}
\end{equation}
\end{multiline}

The equation goes over the width of the pagw, as seen in the screenshot below

I have tried to use the align environment, but it gives me an error saying that the environment cannot be recognised.

Can you help me to resolve this?

The log output file can be seen below

Answer 1

As stated in @DaviCarlisle comment below his answer, your equation has sense, if You insert equal signs at begin of each equation:

\documentclass{article}
\usepackage{amsmath}
\newcommand\E{\mathrm{E}}

\begin{document}

    1. \(\E(Y ) = a\E(X) + b\).   (5 marks)
\begin{align*}
\E(aX+b) & = \sum_{x}(ax+b)p(x)\\
         & = \sum_{X}\bigl(ax\cdot p(x)+b\cdot p(x)\bigr) 
                =  \sum_{X}ax\cdot p(x) + \sum_{X}b\cdot p(x)   \\
         & = a\underbrace{\biggl(\sum_{X}X\cdot p(x)\biggr)}_{E(X)} +
            b\underbrace{\biggl(\sum_{X}p(x)\biggr)}_{1} \\
         & = a\cdot \E(X) + b
\end{align*}
\end{document}

Answer 2

\documentclass{article}

\usepackage{amsmath}

\begin{document}

    1. \(E(Y ) = aE(X) + b\). (5 marks)\par
\begin{multiline}
\begin{equation}
\E(aX+b)&=& \sum_{x}{ax+b)p(x)\\
\sum_{X}(ax\cdot p(x)+b\cdot p(x))\\
\sum_{X}ax\cdot p(x) + \sum_{X}b\cdot p(x)\\
a\underbrace{\left(\sum_{X}X\cdot p(x)\right)}_{E(X)} + b\underbrace{\left(\sum_{X}p(x)\right)}_{1}\\
a\cdot \E(X) + b}
\end{equation}
\end{multiline}
\end{document}

Produces the error

! LaTeX Error: Environment multiline undefined.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.8 \begin{multiline}
                     
? 

After any error the pdf output is not intended to be usable.

The environment here is multline (no i) and should not have equation

But multline does not have & alignment so here it seems you want align*

\documentclass{article}

\usepackage{amsmath}
\newcommand\E{\mathrm{E}}
\begin{document}

    1. \(E(Y ) = aE(X) + b\). (5 marks)
\begin{align*}
\E(aX+b) =& \sum_{x}(ax+b)p(x)\\
&\quad\sum_{X}(ax\cdot p(x)+b\cdot p(x))\\
&\quad\sum_{X}ax\cdot p(x) + \sum_{X}b\cdot p(x)\\
&\quad a\underbrace{\left(\sum_{X}X\cdot p(x)\right)}_{E(X)} + b\underbrace{\left(\sum_{X}p(x)\right)}_{1}\\
&\quad a\cdot \E(X) + b
\end{align*}
\end{document}