Average rate of change on the interval is a slope of the line that goes through the first and last points.
We have 2 points
(2, 8.5)
(4,8.5)
average rate of change= (8.5-8.5)/(4-2)=0/2=0
average rate of change on the interval [5,7]
(5,700) and (7, 100)
(700-100)/(7-5)= (100-700)/2=-600/2=-300
