I'm guessing since the X makes the length of the triangle 2 more than then first one, that it does the same to the other side.
So the height would be 5 and the length 6. When you plug those into the formula A=1/2 (l×w) it makes:
A=1/2(5×6)
A=1/2 (30)
A=15 ft^2.
So the answer is H