maths...(maximize)

If you charge $20 per ticket, you can sell 10,000 tickets to the lacrosse match against Norway. For each dollar increase in price, you sell 1,000 fewer tickets (and for each dollar decrease in price, you sell 1,000 more tickets).

a) what price should you charge to maximize the amount of money yo u receive from ticket sales?

b) how many tickets will you sell at that price, and how much money will you receive?

thx so much