18. The profit in dollars from the sale of x expensive watches is P(x) = 0.06x" - 3x + 7x-4 0.4 -5100. Find the marginal profit when (a) x = 300, (b) x = 2000, (c) x = 6000, and (d) x = 12,000. (a) When x = 300, the marginal profit is $ (Round to the nearest integer as needed.) (b) When x = 2000, the marginal profit is $ (Round to the nearest integer as needed.) (c) When x = 6000, the marginal profit is $ (Round to the nearest integer as needed.) (d) When x = 12,000, the marginal profit is $ (Round to the nearest integer as needed.) 19. Often sales of a new product grow rapidly at first and then level off with time. This is the case with the sales represented by the function S(t) = 100-500t , where t represents time in years. Find the rate of change of sales for the following number of years. a. 4 years The rate of change in sales after 4 years is (Type an integer or decimal rounded to the nearest thousandth as needed.) b. 9 years The rate of change in sales after 9 years is (Type an integer or decimal rounded to the nearest thousandth as needed.)