Need help 158 is not the right answer

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 V 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. N Millions of kilowatts Midnight 4 A.M. 8 A.M. Noon 4 P.M. 8 P.M. Midnight The estimate of the city's total daily usage of electricity for the given day is 158 million kilowatt-hours. (Round to the nearest whole number as needed.)