When a commodity is sold for x dollars per item, the number of items sold is f(x). The item costs 5 dollars to make. The item costs 3 dollars to make. Required:
a. Express the total profit P in terms of x (the answer has been partially filled in for you).
b. What is the rate of change of the total profit as x changes?
c. If 3000 items sell when the price is 45 dollars and if the number of items that are sold decreases by 600 for every 1 dollar increase in price, find the rate of change of the profit when the item price is 45 dollars.