I have attached the problem. Help!
Express the total length of fence needed in terms of x and y. Equate this length to 1500 and express y through x. The total area is xy, so using the obtained expression y(x) the area is xy(x). Find the point of maximum of this function using its derivative.
Well, it's like in hostage situation: you have to give us something in exchange for your demands Disclaimer: it's just a joke to make you smile; don't take it seriously.
However, if you are not able to express the total required fence length in terms of x and y, I am not sure we can help...
Yes, though from the picture and from the answer I think x and y denote the whole width and height, respectively. Thus, 3x + 4y = 1500, from where y = (1500 - 3x) / 4 and the area is indeed x(1500 - 3x) / 4. The maximum point is the same as for x(1500 - 3x) = -3x² + 1500x. The vertex of a parabola ax² + bx + c is at x = -b / 2a, so the maximum point is x = 250.