因为1/x-1/2y=1/3,
所以(2y-x)/2xy=1/3,
所以2y-x=2xy/3,
所以(2x+3xy-4y)/(x-3xy-2y)
=[-2(x-2y)+3xy]/[(x-2y)-3xy]
=(-4xy/3+3xy)/(2xy/3-3xy)
=(5xy/3)/(-7xy/3)
=-5/7.