Matrices & Systems · Hard problem

Let $A = \begin{pmatrix}1 & 2\\0 & 1\end{pmatrix}$ and $B = \begin{pmatrix}1 & 0\\3 & 1\end{pmatrix}$. Because matrix multiplication is not commutative, $(A + B)^2$ need not equal $A^2 + 2AB + B^2$. Find the entry in row 1, column 1 of $(A + B)^2 - A^2 - 2AB - B^2$.