Zero and Identity Matrices and Determinants Practice Test

Let $A=\begin{bmatrix}5 & 0\\ -2 & 1\end{bmatrix}$ and let $O_2=\begin{bmatrix}0&0\0&0\end{bmatrix}$ be the $2\times2$ zero matrix. For matrix $A$, what is $A+O_2$?​

Let $M=\begin{bmatrix}2 & 4\\ 1 & 2\end{bmatrix}$. Does matrix $M$ have an inverse? Why or why not? (Use the determinant to decide.)