Answer
$$AA=\left[\begin{array}{rrr}{1} & {0} & {0} \\ {0} & {4} & {0} \\ {0} & {0} & {9}\end{array}\right] .$$
Work Step by Step
Given
$$
A=\left[\begin{array}{rrr}{-1} & {0} & {0} \\ {0} & {2} & {0} \\ {0} & {0} & {3}\end{array}\right].
$$
We have
\begin{align*}
AA&= \left[\begin{array}{rrr}{-1} & {0} & {0} \\ {0} & {2} & {0} \\ {0} & {0} & {3}\end{array}\right]\left[\begin{array}{rrr}{-1} & {0} & {0} \\ {0} & {2} & {0} \\ {0} & {0} & {3}\end{array}\right]\\
&=\left[\begin{array}{rrr}{1} & {0} & {0} \\ {0} & {4} & {0} \\ {0} & {0} & {9}\end{array}\right]
.
\end{align*}
