Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 19

$5x^3+x^2y-xy^2-4y^3$

Removing the grouping symbols and then combining like terms, the given expression, $ (x^3-y^3)-(-4x^3-x^2y+xy^2+3y^3) ,$ simplifies to \begin{array}{l}\require{cancel} x^3-y^3+4x^3+x^2y-xy^2-3y^3 \\\\= (x^3+4x^3)+(-y^3-3y^3)+x^2y-xy^2 \\\\= 5x^3-4y^3+x^2y-xy^2 \\\\= 5x^3+x^2y-xy^2-4y^3 .\end{array}