The area of the trapezoid is calculated through the equation,
A = ((b₁ + b₂)/2) x h
where b₁ and b₂ are the bases and h is the height. Substituting the known values from the given above,
504 = ((18 + 30)/2) x h
The value of h from the equation is 21. Thus, Molly must make the trapezoid 21 inches high.