The formula for the area of a trapezoid is A=12(b1+b2)hA = \frac{1}{2}(b_1 + b_2)hA=21(b1+b2)h. Solve for hhh.
h=2Ab1+b2h = \frac{2A}{b_1 + b_2}h=b1+b22A
h=Ab1+b2h = \frac{A}{b_1 + b_2}h=b1+b2A
h=2A(b1+b2)h = 2A(b_1 + b_2)h=2A(b1+b2)
h=A2(b1+b2)h = \frac{A}{2(b_1 + b_2)}h=2(b1+b2)A