≈ 63.30 ft²
Heron formula
A = √p(p - a)(p - b)(p - c)
a = 9 ; b = 15 ; c = 20
p = (a + b + c)/2 = (9 + 15 + 20)/2 = 44/2 = 22
A = √22(22-9)(22-15)(22-20)
= √22×13×7×2
= √4004
≈ 63,27
