Answer
(a) $$AB=\left[ \begin {array}{cc} 8&2\\1&-14
\\ -4&18\end {array} \right].$$
(b) $BA$ not defined.
Work Step by Step
Given
$$
A=\left[\begin{array}{rr}{2} & {1} \\ {-3} & {4} \\ {1} & {6}\end{array}\right], \quad B=\left[\begin{array}{rrr}{0} & {-1} & {0} \\ {4} & {0} & {2} \\ {8} & {-1} & {7}\end{array}\right]
$$
We have
(a) \begin{align*}
AB&=\left[\begin{array}{rr}{2} & {1} \\ {-3} & {4} \\ {1} & {6}\end{array}\right]\left[\begin{array}{rrr}{0} & {-1} & {0} \\ {4} & {0} & {2} \\ {8} & {-1} & {7}\end{array}\right]\\
&=\left[ \begin {array}{cc} 8&2\\1&-14
\\ -4&18\end {array} \right].
\end{align*}
(b) $BA$ not defined.