
Area of parallelogram having sides $\begin{aligned}
& \overrightarrow{\mathrm{OA}} & \overrightarrow{\mathrm{OC}}=|\overrightarrow{\mathrm{OA}} \times \overrightarrow{\mathrm{OC}}|=|2 \overrightarrow{\mathrm{a}} \times 3 \overrightarrow{\mathrm{b}}|=15 \
& 6|\overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}|=15 \
& \Rightarrow|\overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}|=\frac{5}{2} \ldots \ldots . .(1)
\end{aligned}$
Area of quadrilateral $\begin{aligned}
& \mathrm{OABC}=\frac{1}{2}\left|\overrightarrow{\mathrm{d}}_1 \times \overrightarrow{\mathrm{d}}_2\right| \
& =\frac{1}{2}|\overrightarrow{\mathrm{AC}} \times \overrightarrow{\mathrm{OB}}|=\frac{1}{2}|(3 \overrightarrow{\mathrm{b}}-2 \overrightarrow{\mathrm{a}}) \times(6 \overrightarrow{\mathrm{a}}+5 \overrightarrow{\mathrm{b}})| \
& =\frac{1}{2}|18 \overrightarrow{\mathrm{b}} \times \overrightarrow{\mathrm{a}}-10 \overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}|=14|\overrightarrow{\mathrm{a}} \times \overrightarrow{\mathrm{b}}| \
& =14 \times \frac{5}{2}=35
\end{aligned}$