I really have no idea where to even begin with this problem. I did some guess and check, but I'm sure there's some method I'm suppose to use that we haven't covered. I understand what it's asking and all, but I'm at a loss as to how to solve it.

Consider the graph of

limited to the domain of (0,9).

Find the largest rectangle that can be constructed inside of that shape, with sides parallel to the x- and y-axes and the right edge of the rectangle being the vertical line x = 9.