Find an equation of the largest sphere with center (5,4,9) that is contained in the first octant?

I really have no clue as to how to set this problem up. I do know the equation for a circle is: (x-h)^2+(y-k)^2+(z-l)^2=r^2

If I substitute these points into the equation I get

(x-5)^2+(y-4)^2+(z-9)^2=r^2

This is as far as I can get. I appreciate the help.