You can add matrices with the same size by adding each element.

(abcd)+(efgh)=(a+eb+fc+gd+h)\begin{pmatrix}a & b \\ c & d\end{pmatrix}+\begin{pmatrix}e & f \\ g & h\end{pmatrix}=\begin{pmatrix}a+e & b+f \\ c+g & d+h\end{pmatrix}