Answer:
20
Explanation:
Easiest way to solve is to consider the formula for the area of the trapezoid, which is 0.5h(a+b), where h is the altitude and a and b are the two bases.
Here we know the area, the altitude and one base so let's sub these in to find the other base:
92=0.5*8*(3+b)
92=4(3+b)
23=4+b
b=20