[-9-6]+[9-9]+[16-20]

You add matrices by individually adding the same row/colums elements. Thus if:

A =
\left |\begin{matrix}
a & b\\
c & d\\
e & f
\end{matrix} \right|

and


B =
\left | \begin{matrix}
g & h\\
i & j\\
k & l
\end{matrix} \right |

Then

A + B =
\left | \begin{matrix}
(a+g) & (b+h)\\
(c+i) & (d+j)\\
(e+k) & (f+l)
\end{matrix} \right |