A paint store carries two brands of latex paint. Sales figures indicate that if the first brand is sold for x₁ dollars per gallon and the second for x2 dollars per gallon, the demand for the first brand will be D₁(x₁, x₂) = 200 − 10x₁ + 20x₂ gallons per month and the demand for the second brand will be D₂(x₁, x₂) = 100 + 5x₁ − 10x₂ gallons per month.

a. Express the paint store’s total monthly revenue from the sale of the paint as a function of the prices x₁ and x₂.

b. Compute the revenue in part (a) if the first brand is sold for $21 per gallon and the second for $16 per gallon.