Question 1204a

Given the vectors ${\mathbf{a}=\begin{pmatrix} 12 \\ - 5 \\ 0 \end{pmatrix}}$ and ${\mathbf{b}=\begin{pmatrix} - 2 \\ 9 \\ - 6 \end{pmatrix},}$ what is the length of projection of ${\mathbf{b}}$ onto ${\mathbf{a}?}$