Since we don't have the measurements for the trapezoid, we can't find the exact cost. However, I can explain how to solve the problem.
First, you need to find the the total area of the pool cover. Since it is in the shape of a trapezoid, use the formula below.
A = 0.5(h)(the sum of the bases)
Once you have the area, multiply by 0.79 and you will have the cost.