The graph shows a city's daily rate of use of electricity (in millions of kilowatts). estimate the total daily usage of electricity by using rectangles whose height is given by the value of the function at the left side of each rectangle. let the width of each rectangle be 2 hours.

This is a long and tedious process but basically what you want to do is draw little rectangles to approximate the area. In this case what the problem wants you to do is
f(0)*2(length times width)+f(2)*2+2(4)*2+f(6)*2+f(8)*2+f(10)*2+f(12)*2+... +f(20)*2+f(22)*2=
(f(0)+f(2)+f(4)+f(6)+f(8)+f(10)+f(12)+f(14)+f(16)+f(18)+f(20)+f(22))*2=
approximately, I don't know how specific you're teacher wants them but Im putting down f(x) in integers for the sake of simplicity:
(9+10+11+12+15+18+21+26+27+26+17+13)*2=410

2,4,5,8,10 are divisible by 40