Working out the area of a triangle within another triangle

Question

The area of $\triangle OBF$ is $5$ cm$^2$. $F$ is a point on $OA$ such that $OF = \dfrac9{11}OA$. Work out the area of $\triangle OBA$.

I have tried to solve this problem for a while, and just can't figure out how to do it. Please help.

Answer 1

Hint: Observe that both $\triangle OBF$ and $\triangle AOB$ have same height on bases $OF$ and $AO$

Here DE is perpendicular for $\triangle DCB$