I do not understand how to come to the answer to the second part

company produces two types of solar panels per year: x thousand of type A and y thousand of type B. The revenue and cost equations, in millions of d R(x,y) = 3x + 4y C(x.y) = x2 - 3xy + 7y2 + 12x -57y -5 Determine how many of each type of solar panel should be produced per year to maximize profit. The company will achieve a maximum profit by selling 3000 solar panels of type A and selling 5000 solar panels of type B. The maximum profit is $ million