The average cost of online photos decreases from 0.50 per print to $0.15 per print between 2002 and 2009. find the average rate of change in the cost. explain what it means

The average rate of changeis given by:
 m = (y2-y1) / (x2-x1)
 m = (0.50-0.15) / (2002-2009)
 Rewriting:
 m = (0.35) / (- 7)
 m = -0.05 $ / year
 
 the average rate of change in the cost is:
 m = -0.05 $ / year
 It means that the cost per print decreases 0.05 $ per year

X equals about 2.3,