The linear function $y = 2 x + 28$ models the average cost of a haircut in a certain city, where $y$, is the average price of a haircut $x$ years after 1985. Find the slope for the model. Then describe what this means in terms of the rate of change in the average cost of a haircut over time.

a) $m =$

b) The average cost of a haircut in this city (Enter: increased or decreased or not changed ) at a rate of per year after 1985.