Hi Christine -
In geometry, you probably learned that the area of a trapezoid equals:
A = (1/2) x (B1 + B2) x h
This formula has four variables (A, B1, B2 and h) and if you are given the value for three of the four variables, you can easily solve for the missing fourth variable.
In your problem, you have been given 3 of the 4 variables (B1, B2 and A), so substitute them into the equation and solve for h.
Hope this helps