Diagonalization & Matrix Powers · Easy problem

The matrix $A = \begin{pmatrix}5 & -2\\0 & 1\end{pmatrix}$ is diagonalized as $A = PDP^{-1}$ with $D = \begin{pmatrix}5 & 0\\0 & 1\end{pmatrix}$ and $P = \begin{pmatrix}1 & 1\\0 & a\end{pmatrix}$. Find $a$.