Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 284: 35

$-6a^4-8ab+7ab^2$

Removing the grouping symbols and then combining like terms, the given expression, $ (3a^4-5ab+6ab^2)-(9a^4+3ab-ab^2) ,$ simplifies to \begin{array}{l}\require{cancel} 3a^4-5ab+6ab^2-9a^4-3ab+ab^2 \\\\= (3a^4-9a^4)+(-5ab-3ab)+(6ab^2+ab^2) \\\\= -6a^4-8ab+7ab^2 .\end{array}