3/x-2/y=3
所以x!=0,y!=0
所以xy!=0,原式分子分母同除xy得,
(2x-3y-xy)/(7xy+9y-6x)
=(2/y-3/x-1)/(7+9/x-6/y)
=(-(3/x-2/y)-1)/(7+3(3/x-2/y))
=(-3-1)/(7+3*3)
=-4/16
=-1/4