Molly is making a sign in the shape of a trapezoid. One base is 18 inches and the other is 30 inches. How high must she make the sign so its area is 504 square inches?

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.