n = 4 means that you are going to divide the area in 4 equal parts, that means that the size of the intervals is [20-4] / 4 = 16 / 4 = 4.
Now you can estimate the area from 4 to 20, by adding the area from 4 to 8 + the area from 8 to 12 + the area from 12 to 16 + the area from 16 to 20
And estimate each small area by a rectangle or a trapezoid.
As a trapezoid, area of each region is [f(x) + f(x+4)] /2 [x +4 - x] = [f(x) + f(x+4)] /2 [4] =
=[f(x) + f(x+4)] / 8
Area from 4 to 8: [11 +25] / 8 = 4.50
Area from 8 to 12: [25+16]/8 = 5.125
Area from 12 to 16 = [16+9]/8 = 3.125
Area from 16 to 20 = [9+31]/8 = 5.00
Total area = 17.75
Answer: 17.75