Use the Pythagorean theorem to find the common side of the top triangle. If I read your dimensions correctly, the larger triangle has one side of 36 and a hypotenuse of 39. Find the third (common) side:
X2 + 362 = 392
X = 15
Area = 1/2BH = (1/2)(15)(36) = 270 in2
We now also have the information for the bottom triangle:
Area = (1/2)*(15)(8) = 60 in2
[Just to check, 82 + 152 = 172? Yes, it does.]
Bob
8 in.