Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 14

$-6x^4y^2-13x^2y^2+6xy$

Removing the grouping symbols and then combining like terms, the given expression, $ (6x^4y^2-10x^2y^2+7xy)+(-12x^4y^2-3x^2y^2-xy) ,$ simplifies to \begin{array}{l}\require{cancel} 6x^4y^2-10x^2y^2+7xy-12x^4y^2-3x^2y^2-xy \\\\= (6x^4y^2-12x^4y^2)+(-10x^2y^2-3x^2y^2)+(7xy-xy) \\\\= -6x^4y^2-13x^2y^2+6xy .\end{array}