A printed poster is to have a total area of 735 square inches with top and bottom margins of 4 inches and side margins of 6 inches. What should be the dimensions of the poster so that the printed area be as large as possible?

To solve this problem let x denote the width of the poster and let y denote the length. We need to maximize the following function of x and y:
for this I got (x-12)(y-8)
the next step is this. We can reexpress this as the following function of x alone:
F(x)=?

Question

A printed poster is to have a total area of 735 square inches with top and bottom margins of 4 inches and side margins of 6 inches. What should be the dimensions of the poster so that the printed area be as large as possible?

To solve this problem let x denote the width of the poster and let y denote the length. We need to maximize the following function of x and y: 
for this I got (x-12)(y-8)
the next step is this. We can reexpress this as the following function of x alone:
F(x)=?