Diagonalization & Matrix Powers · Easy problem

A matrix has eigenvectors $\mathbf{v}_1 = \begin{pmatrix}1\\1\end{pmatrix}$ and $\mathbf{v}_2 = \begin{pmatrix}1\\-1\end{pmatrix}$. Write $\mathbf{x} = \begin{pmatrix}5\\1\end{pmatrix}$ in this eigenbasis as $\mathbf{x} = c_1\mathbf{v}_1 + c_2\mathbf{v}_2$. Find $c_1$.