Answer
$ \left[\begin{array}{rrr}
-2 & -7 & 7\\
10 & -2 & 7
\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.
---
$\left[\begin{array}{lll}
6 & -9 & 2\\
4 & 1 & 3
\end{array}\right]+\left[\begin{array}{lll}
-8 & 2 & 5\\
6 & -3 & 4
\end{array}\right]=\qquad$ ... dimensions match, continue
$=\left[\begin{array}{lll}
6+(-8) & -9+2 & 2+5\\
4+6 & 1+(-3) & 3+4
\end{array}\right]$
$=\left[\begin{array}{rrr}
-2 & -7 & 7\\
10 & -2 & 7
\end{array}\right]$
