若x-y/y=2,求代数式x-y/x-2(x-y)/x+y的值

题目

若x-y/y=2,求代数式x-y/x-2(x-y)/x+y的值
今晚要······越快越好,做不出来了······

答案

(x-y)/y=2
x/y-1=2
x=3y
(x-y)/x-2(x-y)/(x+y)
=(3y-y)/(3y)-2(3y-y)/(3y+y)
=2/3-1
=-1/3