Question 1204a

Given the vectors ${\mathbf{a}=\begin{pmatrix} 1 \\ - 8 \\ - 4 \end{pmatrix}}$ and ${\mathbf{b}=\begin{pmatrix} 4 \\ 8 \\ - 1 \end{pmatrix},}$ what is the length of projection of ${\mathbf{b}}$ onto ${\mathbf{a}?}$