Answer
$ \left[\begin{array}{rr}
6 & 6\\
-12 & 9
\end{array}\right]$
Work Step by Step
1. Check if the matrices have the same dimension. If they don't, the sum/difference is not defined.
2. The sum difference will be a matrix with the same dimension.
3. Position by position, add/subtract the corresponding entries.
---
$\left[\begin{array}{ll}
9 & 4\\
-8 & 2
\end{array}\right]+\left[\begin{array}{ll}
-3 & 2\\
-4 & 7
\end{array}\right]=\qquad$ ... dimensions match, continue
$=\left[\begin{array}{ll}
9+(-3) & 4+2\\
-8+(-4) & 2+7
\end{array}\right]$
$=\left[\begin{array}{rr}
6 & 6\\
-12 & 9
\end{array}\right]$
