A company is planning to spend $10,000 on advertising. It costs $3,000 per minute to advertise on television and $1,000 per minute to advertise on radio. If the firm buys x minutes of television advertising and y minutes of radio advertising, then its revenue in thousands of dollars is given by f (x, y) = -2x2 - y2 + xy + 8x + 3y. How can the firm maximize its revenue?