I redraw this picture

If this two squares don't overlap, the sum their length is:

12 + 12 = 24 [units]

So the new length of rectange lost:

24 - 20 = 4 [units]

=> MN = PQ = 4

=> AM = BN = DP = CQ = \(\dfrac{20-4}{2}=8\)

So the area of \(\Delta BCQ\) is:

\(\dfrac{12\cdot8}{2}=48\left(units^2\right)\)