已知1/x-1/y=2,求3x+7xy-3y/2x-3xy-2y
答案是
1/x-1/y=2
(y-x)/xy=2得：x-y=-2xy
(3x+7xy-3y)/(2x-3xy-2y)
=[3(x-y)+7xy]/[2(x-y)-3xy]
=(-6xy+7xy)/(-4xy-3xy)
=xy/(-7xy)
=-1/7

谁能帮我解释一下
=[3(x-y)+7xy]/[2(x-y)-3xy]
=(-6xy+7xy)/(-4xy-3xy)

是怎么变化的.请详细的说明一下.