xy-4x-6y=0
(x-6)(y-4)=24
x-6=±(1,2,3,4,6,8,12,24) x≠6
y-4=±(24,12,8,6,4,3,2,1) y≠4
∴max(x-y)=25 (x-6=24,y-2=1)