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: