Matrices

Let w = $\begin{pmatrix} 2 \\ -1 \end{pmatrix}$. There exists a $2 \times 2$ matrix ${P}$ such that $\text{proj}_{{w}} {v} = {P} {v}$ for all 2-dimensional vectors ${v}$. Find ${P}$.

I'm not really sure where to start for this problem. Should I plug w into the projection formula?