(x^{-3}+ y^{-3}) / (x^{-2}– x^{-1}y^{-1}+ y^{-2})

the answer is (x + y) / xy .

p.s '/' indicated fraction.

I tried it for almost 45 minutes but stuck at the last part.

I get this : [x+y (x^{2}-xy-y^{2})] / [xy (x^{2}+y^{2})]

This is my first post so please prove this forum worth staying !!!