Where possible, find each product.
a. $\left[\begin{array}{rr}1 & 0 \\ 0 & -1\end{array}\right]\left[\begin{array}{rr}-1 & 0 \\ 0 & -1\end{array}\right]$
b. $\left[\begin{array}{rr}-1 & 0 \\ 0 & -1\end{array}\right]\left[\begin{array}{rrr}-1 & 0 & 1 \\ 0 & -1 & 1\end{array}\right]$