Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 13

$-11x^4y^2-11x^2y^2+2xy$

Removing the grouping symbols and then combining like terms, the given expression, $ (7x^4y^2-5x^2y^2+3xy)+(-18x^4y^2-6x^2y^2-xy) ,$ simplifies to \begin{array}{l}\require{cancel} 7x^4y^2-5x^2y^2+3xy-18x^4y^2-6x^2y^2-xy \\\\= (7x^4y^2-18x^4y^2)+(-5x^2y^2-6x^2y^2)+(3xy-xy) \\\\= -11x^4y^2-11x^2y^2+2xy .\end{array}