PLEASE HELP :)

Mrs. Bell's class is selling Hobbs Middle Jaguar t-shirts to raise money for a trip. They typically sell 400 t-shirts a year for $10 per shirt. Mrs bell knows that for every $1 increase in price, they will sell 20 less t-shirts. What is the revenue function R(x), where x represents the $1 price increases and revenue is equal the number of t-shirts sold times the cost of each shirt