若1/x-1/y=3,那么：
(y-x)/(xy)=3,即y-x=3xy
所以：x-y=-3xy
所以：(x-3xy-y)/(3x-2xy-3y)
=(x-y-3xy)/[3(x-y)-2xy]
=(-3xy-3xy)/(-9xy-2xy)
=(-9xy)/(-11xy)
=9/11