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 - 15x1 + 30x2 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.