Need help with part a and part b

Suppose the total cost C(x) (in dollars) to manufacture a quantity x of weed killer (in hundreds of liters) is given by the function C(x) =x - 4x + 7x + 40, where x > 0. a) Where is C(x) decreasing? b) Where is C(x) increasing? a) Select the correct choice below and fill in any answer boxes within your choice. O A. The function is decreasing on the open interval(s) (Type your answer in interval notation. Simplify your answer. Use integers or fractions for any numbers in the expression. Use a comma to separate answers as needed.) O B. The function is never decreasing. b) Select the correct choice below and fill in any answer boxes within your choice. O A. The function is increasing on the open interval(s) (Type your answer in interval notation. Simplify your answer. Use integers or fractions for any numbers in the expression. Use a comma to separate answers as needed.) O B. The function is never increasing