How to put a box in aligned equations?

Question

Can someone help me with the following question?

I'm writing an equation like this:

\begin{align*}
   a =& b - c \\
     -& d - e
\end{align*}

How can I put a box around the part

b - c \\
d - e

Thanks.

Answer 1

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[\begin{array}{r@{}|l|}
\cline{2-2}
a =& b - c \\
     -& d - e\\
\cline{2-2}
\end{array}
\]
\end{document}