The decrease per year in home sales between 2 years is the same as the slope of the graph between those two points, which is defined as ?y/?x. In 1987 the number of sales is 156,000, and in 1990 it is 114,000. The difference between these two numbers is 42,000, or change in y. Divide this number by the change in x, or years, which is 1990-1987 = 3. 42000/3=14000, which is the decrease per year in the number of homes sold between 1987 and 1990, making C the right answer.