Solve this

If $A=\left[\begin{array}{ll}1 & -1 \\ 2 & -1\end{array}\right], B=\left[\begin{array}{ll}a & -1 \\ b & -1\end{array}\right]$ and $(A+B)^{2}=\left(A^{2}+B^{2}\right)$ then find the values of $a$ and $b$.