Question 1204a

Given the vectors ${\mathbf{a}=\begin{pmatrix} - 4 \\ 1 \\ - 8 \end{pmatrix}}$ and ${\mathbf{b}=\begin{pmatrix} 9 \\ - 2 \\ - 6 \end{pmatrix},}$ what is the length of projection of ${\mathbf{a}}$ onto ${\mathbf{b}?}$