Simplify, using only positive exponents in your answer.
x^-4+y^-4/x^-3+y^-3
(x^-4 + y^-4) / (x^-3 + y^-3)
(y^4 + x^4)/(x^4*y^4) / (y^3 + x^3)/(x^3*y^3)
(x^4 + y^4)/(x^4*y^4) * (x^3*y*3)/(x^3 + y^3)
(x^4 + y^4)/(xy(x^3+y^3))
x^-4+y^-4/x^-3+y^-3
(y^4 + x^4)/(x^4*y^4) / (y^3 + x^3)/(x^3*y^3)
(x^4 + y^4)/(x^4*y^4) * (x^3*y*3)/(x^3 + y^3)
(x^4 + y^4)/(xy(x^3+y^3))