My answer is not displayed correctly. The internet archive has a correct version of the first revision, see this screenshot from archive. The difference is that now, though I didn't do any changes to that part, it says
\[
M=
\begin{bmatrix}
1 & 2 & 3 & 4 & 5\ 3 & 4 & 5 & 6 & 7\ \end{bmatrix}
\]
instead of
\[
M=
\begin{bmatrix}
1 & 2 & 3 & 4 & 5\\
3 & 4 & 5 & 6 & 7\\
\end{bmatrix}
\]
Why?
