xy/(x+y)=2
(x+y)/xy=1/2
1/x+1/y=1/2
(3x-xy+3y)/(-x+3xy-y)
=(3/y-1+3/x)/(-1/y+3-1/x)
=[3(1/x+1/y)-1]/[(3-(1/x+1/y)]
=(3/2-1)/(3-1/2)
=(1/2)/(5/2)
=1/5