Based on your graph, I think you mean quadrilateral PEFQ.
The way to hack this problem is that you shall find two pair of "similar" triangulars
triangle AFQ is same shape as FRC by a ratio of 4/3 : 1
triangle AEP is same shape as ERC by a ratio of 1/3 : 1/2
By knowing this, you can calculate the height of AFQ and AEP, and therefore, calculate the area of those two triangular.
height of AFQ = (4/3)/(4/3 + 1)* length of CB
height of AEP = (1/3)/(1/3+1/2)* length of CB
area of quadrilateral PEFQ = area of AFQ - area of AEP