An accessories company finds that the cost, in dollars, of producing x belts is given by C(x) = 790 + 36x - 0.066x. Find the rate at which average cost is changing when 170 belts have been produced. First, find the rate at which the average cost is changing when x belts have been produced. C'(x) = When 170 belts have been produced, the average cost is changing at (Round to four decimal places as needed.) for each additional belt. dollars per belt dollars belts per dollar belts