the area of a triangle is 1/2 bh
so to find the area of the larger triangle do the following calculation:
1/2 time 12 time 7. that will give you 42 ft squared. that is the larger triangle.
the next way to find the area i will show you is:
pretend that the the triangle is inside of a rectangle. now you have 4 triangles. you know that the base of the rectangle is 15 ft anf the height is 7. now take the smaller triangle and it has its rectangle base 3 and height 7. multiply to get 21 feet squared. divide by 2 since there are 2 triangles in the small rectangle. 10.5 feet squared