Orthogonality & Least Squares · Easy problem

The vectors $\mathbf u_1 = (1, 1)$ and $\mathbf u_2 = (1, -1)$ form an orthogonal basis of $\mathbb R^2$. Write $\mathbf b = (5, 3)$ as $\mathbf b = c_1\mathbf u_1 + c_2\mathbf u_2$ and find $c_2$.