The marketing division of a large firm has found that it can model the sales generated by an advertising campaign as S(u) = 0.75vu + 1.7 millions of dollars when the firm invests u thousand dollars in advertising. The firm plans to invest u(x) = -2.4x2 + 53x + 100 thousand dollars each month where x is the number of months after the beginning of the advertising campaign. (a) Write the function for the model that gives the sales in million dollars generated when x is the number of months since the beginning of the ad campaign. S( u (x ) ) = million dollars (b) Write the formula for the rate of change of predicted sales x months into the campaign. ds dx million dollars per month (c) What will be the rate of change of sales when x = 19? (Round your answer to three decimal places.) $ million per month