Diagonalization & Matrix Powers · Easy problem

The symmetric matrix $A = \begin{pmatrix}3 & 1\\1 & 3\end{pmatrix}$ is orthogonally diagonalized as $A = QDQ^{T}$ with $D = \begin{pmatrix}d_1 & 0\\0 & d_2\end{pmatrix}$ and $d_1 \ge d_2$. Find $d_1$.