use the linear slope formula to solve for m:
m=(y2-y1)/(x2-x1)
using the points (-1,1) and (1,5) you can find the slope for the first interval:
m=(5-1)/(1-(-1))
m=4/2
m=2
you can double check by using the same formula on the second interval, which is (1,5) to (3,9).
m=(9-5)/(3-1)
m=4/2
m=2
therefore, the rate of change is 2.
