x+y分之xy=3,
xy/(x+y)=3,
xy=3(x+y)
-x+3xy-y分之2x-3xy+2y
=(2x-3xy+2y)/(-x+3xy-y)
=[2x-9(x+y)+2y]/[-x+9(x+y)-y]
=-7(x+y)/8(x+y)
=-7/8