Vector Algebra

Let $\overrightarrow{O A}=2 \vec{a}, \overrightarrow{O B}=6 \vec{a}+5 \vec{b}$ and $\overrightarrow{O C}=3 \vec{b}$, where $O$ is the origin. If the area of the parallelogram with adjacent sides $\overrightarrow{O A}$ and $\overrightarrow{O C}$ is 15 sq. units, then the area (in sq. units) of the quadrilateral $O A B C$ is equal to: