In order to find the rate of change of the resale value of automobile, use the following formula:

where (x1,y1) and (x2,y2) are any two points on the line.
For instance, based on the given graph, you can use:
(x1,y1) = (0 , 30000)
(x2,y2) = (2 , 15000)
Replace the previous values into the formula for m and simplify:

Hence, the rate of change is -7500