3/x -2/y=3
等式两边同乘以xy
3y-2x=3xy
(2x-3y-xy)/(7xy+9y-6x)
=[-(3y-2x)-xy]/[3(3y-2x)+7xy]
=(-3xy-xy)/[3·(3xy)+7xy]
=(-4xy)/(16xy)
=-1/4