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