A) You know the formula for the area of a trapezoid is ...
A = (1/2)(b₁ +b₂)×h
B) You are given the area and the lengths of the two bases. Substituting the given numbers into the formula will give you an equation for h.
280 cm² = (1/2)(8 cm +20 cm)×h
C) Solving the equation of part B, we have
280 cm² = (14 cm)×h
280 cm²/(14 cm) = h . . . . . . . . divide by the coefficient of h
20 cm = h . . . . . . . . . . . . . . . .. evaluate
The height of the trapezoid is 20 cm.