​ monopolist’s profit-maximizing output

Currently, a​ monopolist’s profit-maximizing output is 250 units per week. It sells its output at a price of ​$50 per unit and collects ​$20 per unit in revenues from the sale of the last unit produced each week. The​ firm’s total costs each week are ​$9,000.

a. What are the firm’s weekly economic profits?

b. What is the firm’s marginal cost?

c. What is the firm’s average total cost?