Question 1204a

Given the vectors ${\mathbf{a}=\begin{pmatrix} - 8 \\ 4 \\ - 1 \end{pmatrix}}$ and ${\mathbf{b}=\begin{pmatrix} 0 \\ 5 \\ 12 \end{pmatrix},}$ what is the length of projection of ${\mathbf{a}}$ onto ${\mathbf{b}?}$