1.5: Triangle with extended lines meeting at a point

The diagram shows a triangle $OAB$ such that $\overrightarrow{OA} = \mathbf{a}$ and $\overrightarrow{OB} = \mathbf{b}$. The point $P$ lies on $OA$ such that $OP = \frac{3}{4}OA$. The point $Q$ is the mid-point of $AB$. The lines $OB$ and $PQ$ are extended to meet at the point $R$. Find, in terms of $\mathbf{a}$ and $\mathbf{b}$,