Answer
(a) $$AB= \left[ \begin {array}{cc} -1&19\\ 4&-27
\\ 0&14\end {array} \right] .$$
(b) $BA$ not defined.
Work Step by Step
Given
$$
A=\left[\begin{array}{rr}{-1} & {3} \\ {4} & {-5} \\ {0} & {2}\end{array}\right], \quad B=\left[\begin{array}{ll}{1} & {2} \\ {0} & {7}\end{array}\right]
$$
We have
(a) \begin{align*}
AB&=\left[\begin{array}{rr}{-1} & {3} \\ {4} & {-5} \\ {0} & {2}\end{array}\right] \left[\begin{array}{ll}{1} & {2} \\ {0} & {7}\end{array}\right] \\
&= \left[ \begin {array}{cc} -1&19\\ 4&-27
\\ 0&14\end {array} \right]
.
\end{align*}
(b) $BA$ not defined.