A chain of paint store carries two brands of latex paint. Sales figures indicate that if the first brand it sold for x dollars per gallon and the second for y dollars per gallon, the demand for the first brand will be D1(x, y)=400−20x+80y gallons per month and the demand for the second brand will be D2(x, y)=300+70x−10y gallons per month. (a) Express the chain of paint store's monthly revenue from the sale of paint as a function of x and y. R(x, y)= (b) Compute the revenue from part (a) if the first brand it sold for $11 per gallon and the second for $14 per gallon.