The area A of a trapezoid with height h and bases b₁ and b₂ is equal to the average of the bases times the height:
A = (b₁ + b₂) h / 2
We're given A = 864, h = 24, and one of the bases has length 30, so
864 = (b₁ + 30) 24 / 2
864 = (b₁ + 30) 12
864 = (b₁ + 30) 12
72 = b₁ + 30
b₁ = 42
