Let O A =2 a, O B =6 a +5 b and O C =3 b, where O is the origin. If the area of the parallelogram with adjacent sides O A and O C is 15 sq. units,…

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