The answer is h = 2A/(b1 + b2).
In order to find this, use all the methods for solving typically used in these types of equations. See the below example.
A = 1/2(b1 + b2)h ---> Multiply both sides by 2.
2A = (b1 + b2)h -----> Divide both sides by (b1 + b2)
2A/(b1 + b2) = h