Orthogonality & Least Squares · Easy problem

Write $\mathbf b = (2, 3, 4)$ as $\mathbf b = \mathbf p + \mathbf r$, where $\mathbf p$ is a multiple of $\mathbf a = (1, 0, 1)$ and $\mathbf r$ is orthogonal to $\mathbf a$. Find $\|\mathbf r\|$.