The rate of change is referred as the slope (or gradient) of a line. Where slope, m, equals (y2-y1)/(x2-x1).
Pick any two points on the line and plug into your formula.
(x1,y1)=>(1,28)
(x2,y2)=>(2,36)
m=(36-28)/(2-1) => 8/1
Thus the slope (rate of change/gradient) is 8.