Find the following matrices where $A=\left[\begin{array}{ll}9 & 2 \\ 6 & 6\end{array}\right]$ and $B=\left[\begin{array}{ll}1 & 8 \\ 1 & 3\end{array}\right]$. a. $A+B$ b. $A-B$ c. $-5 \mathrm{~A}$ d. $3 \mathrm{~A}+4 \mathrm{~B}$ a. Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. $A+B=\square$ (Simplify your answer.) B. This matrix operation is not possible.

Step 1 ：Given matrices A and B as $A=\left[\begin{array}{ll}9 & 2 \ 6 & 6\end{array}\right]$ and $B=\left[\begin{array}{ll}1 & 8 \ 1 & 3\end{array}\right]$ respectively.

Step 2 ：To find the sum of the matrices A and B, we perform matrix addition. This is done element by element. That is, the element in the i-th row and j-th column of the resulting matrix is the sum of the elements in the i-th row and j-th column of the two matrices being added.

Step 3 ：Adding the corresponding elements of matrices A and B, we get $A+B=\left[\begin{array}{ll}10 & 10 \ 7 & 9\end{array}\right]$

Step 4 ：Final Answer: \(\boxed{\left[\begin{array}{ll}10 & 10 \\ 7 & 9\end{array}\right]}\)