Exercise 11.2.6

Suppose $B=\left\{\mathbf{u}_{1},\mathbf{u}_{2},\mathbf{u}_{3}\right\}$ is an orthogonal basis of $\mathbb{R}^{3}$. We have been told that

but it is not known what $\mathbf{u}_{2}$ and $\mathbf{u}_{3}$ are. Find the first coordinate of the vector

with respect to the basis $B$.