Check the picture below.
so we have two rectangles and one triangle, as you see there.
now, the large rectangle is just a 10x8, so that's 80 ft².
the small rectangle is just a 4x4, so that's 16 ft².
the small triangle, notice, it has a right-angle, thus it has a height of 4, and a base of 2.
recall A = (1/2)bh, so the area of the triangle is (1/2)(2)(4) = 4 ft².
80 + 16 + 4 = 100 ft².