Symmetric Matrices & Quadratic Forms · Medium problem

A symmetric matrix $A$ has eigenvalue $3$ with unit eigenvector $\mathbf u = \tfrac{1}{\sqrt2}\begin{pmatrix}1\\1\end{pmatrix}$ and eigenvalue $7$ with unit eigenvector $\mathbf w = \tfrac{1}{\sqrt2}\begin{pmatrix}1\\-1\end{pmatrix}$. Find the entry of $A$ in row 1, column 2.