Heres the problem:

Find the maximum possible volume of a rectangular box that has its base in the xy-plane and its upper vertices on the elliptic paraboloid z = 9 - x^2 - y^2

So we are trying to maximize V = xyz.

I need 2 constraints. One is: z = 9 - x^2 - y^2

Can someone help me find the other? I know it has to do with base in xy plane but I cant derive an equation out fo that.