Question 1204a

Given the vectors ${\mathbf{a}=\begin{pmatrix} - 3 \\ 6 \\ - 2 \end{pmatrix}}$ and ${\mathbf{b}=\begin{pmatrix} 9 \\ - 2 \\ 6 \end{pmatrix},}$ what is the length of projection of ${\mathbf{a}}$ onto ${\mathbf{b}?}$