The general formula for the projection of a vector
\[\mathbf{x}\]
onto a vector \[\mathbf{y}\]
is\[Proj_{\mathbf{y}} \mathbf{x} = \frac{\mathbf{x} \cdot \mathbf{y}}{| \mathbf{y} |} \mathbf{y}\]
Example: Take
\[\mathbf{x} = \begin{pmatrix}1\\0\\2\\3\end{pmatrix} \: \mathbf{y} = \begin{pmatrix}-3\\2\\1\\0\end{pmatrix}\]
\[\mathbf{x} \cdot \mathbf{y} = \begin{pmatrix}1\\0\\2\\3\end{pmatrix} \cdot \begin{pmatrix}-3\\2\\1\\0\end{pmatrix}=-3+0+2+0=-1\]
\[| \mathbf{y} | = \sqrt{(-3)^2+2^2+1^2+0^2} = \sqrt{14}\]
Then
\[Proj_{\mathbf{y}} \mathbf{x} = \frac{-1}{\sqrt{14}} = - \frac{\sqrt{14}}{14}\]