If $A=\left[\begin{array}{ccc}2 & 3 & -1 \\ 1 & 4 & 2\end{array}\right]$ and $B=\left[\begin{array}{ll}2 & 3 \\ 4 & 5 \\ 2 & 1\end{array}\right]$, then $\mathrm{AB}$ and $\mathrm{BA}$ are defined and equal.