9. The marginal cost to produce t-shirts is $3.00/t-shirt and the fixed cost is $400. The price for 20 t-shirts is $12/shirt. The price for 60 shirts is $10/t-shirt. (10 marks) a. Find the cost, revenue, and profit functions. b. Find the derivative of the profit function and interpret the derivative when you have 35 t-shirts. c. How many t-shirts need to be sold to maximize the profit. What is the price and profit when you have a maximum. d. Use the second derivative test to confirm you have a maximum