8y and then I solved for y in the area equation and got y=8,000/x then I put that into the other equation and got 20x+64,000/x then took the derivative and got 20-64,000x^-2 and when I solved for the derivative equals 0 I got 40 radical 2 but my teacher said it is supposed to be 20 radical 15, so I do not know where I went wrong.