\begin{exercise}{M1}{Multiplying matrices}{0010}
\begin{exerciseStatement}
Of the following three matrices, only two may be multiplied. \[
A= \left[\begin{array}{cccc}
1 & 1 & -1 & 2 \\
0 & 1 & 2 & 0
\end{array}\right] \hspace{1em} B= \left[\begin{array}{cc}
1 & 2 \\
2 & 5 \\
2 & 0
\end{array}\right] \hspace{1em} C= \left[\begin{array}{cccc}
1 & 2 & 5 & 6 \\
0 & 1 & 2 & 3 \\
-1 & -2 & -4 & -5
\end{array}\right] \] Explain which two can be multiplied and why. Then show how to find their product.
\end{exerciseStatement}
\begin{exerciseAnswer}\[BA= \left[\begin{array}{cccc}
1 & 3 & 3 & 2 \\
2 & 7 & 8 & 4 \\
2 & 2 & -2 & 4
\end{array}\right] \]\end{exerciseAnswer}
\end{exercise}