Orthogonality & Least Squares · Easy problem

The vectors $\mathbf u_1 = (1, 1, 0)$ and $\mathbf u_2 = (1, -1, 1)$ are orthogonal and span a plane $W$. Let $\hat{\mathbf b}$ be the orthogonal projection of $\mathbf b = (3, 1, 2)$ onto $W$. Find the third entry of $\hat{\mathbf b}$.