Given:
Area of the trapezoid is 39 ft².
The image contains a measurement of 11 ft and 15 ft on its bases.
Area of a trapezoid = (a + b)/2 * h
39 ft² = (11ft + 15ft)/2 * h
39ft² = 26ft/2 * h
39ft² = 13ft * h
39ft² ÷ 13ft = h
3ft = h.
The height of the trapezoid is 3 feet.