Special products and factoring

Given that x+y+xy=1, where x and y are nonzero real numbers, find the value of xy+1/xy-y/x-x/y.

Thus,
xy+1/xy-y/x-x/y
= [xy+xy]*[1+1]/xy
=2xy*2/xy
4xy
/xy
=4.

Question:
How did it became [xy+xy]*[1+1]/xy?