Answer
(a) $AB$ not defined.
(b) $$BA=\left[ \begin {array}{cc} 9&-4\\ 17&3
\\ -1&-8\end {array} \right] .$$
Work Step by Step
Given
$$A
=\left[\begin{array}{rr}{2} & {-3} \\ {5} & {2}\end{array}\right], \quad B=\left[\begin{array}{rr}{2} & {1} \\ {1} & {3} \\ {2} & {-1}\end{array}\right]
$$
We have
(a) $AB$ not defined.
(b)\begin{align*}
BA&=\left[\begin{array}{rr}{2} & {1} \\ {1} & {3} \\ {2} & {-1}\end{array}\right] \left[\begin{array}{rr}{2} & {-3} \\ {5} & {2}\end{array}\right] \\
&= \left[ \begin {array}{cc} 9&-4\\ 17&3
\\ -1&-8\end {array} \right] .
\end{align*}
