Got stuck on these practice problems.

The profit per acre from a grove of orange trees is given by x(170 - x) dollars, where x is the number of orange trees per acre. How many trees per acre will maximize the profit? trees per acre Analysis of daily output of a factory during a 8-hour shift shows that the hourly number of units y produced after t hours of production is y = 70t+ _+2 - +3, outs8. (a) After how many hours will the hourly number of units be maximized? hr (b) What is the maximum hourly output? units/hr An inferior product with a large advertising budget sells well when it is introduced, but sales fall as people discontinue use of the product. Suppose that the weekly sales S are given by 500t S = - t20 (t + 8)2 where S is in millions of dollars and t is in weeks. After how many weeks will sales be maximized? weeks