Q11 (c)

In the Argand diagram below, $\overrightarrow{OA}=z_1$, $\overrightarrow{OB}=z_2$, $OA=OB$ and $\overrightarrow{AC}=\overrightarrow{CB}$. Express $\overrightarrow{OC}$ in terms of $z_1$ and $z_2$.