Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 20

$7x^3-x^2y+xy^2-3y^3$

Removing the grouping symbols and then combining like terms, the given expression, $ (x^3-y^3)-(-6x^3+x^2y-xy^2+2y^3) ,$ simplifies to \begin{array}{l}\require{cancel} x^3-y^3+6x^3-x^2y+xy^2-2y^3 \\\\= (x^3+6x^3)+(-y^3-2y^3)-x^2y+xy^2 \\\\= 7x^3-3y^3-x^2y+xy^2 \\\\= 7x^3-x^2y+xy^2-3y^3 .\end{array}