Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 285: 89

$17a+3x-21$

Using the Distributive Property and then combining like terms, the given expression, $ 2a-3(-x-5a+7) ,$ simplifies to \begin{array}{l}\require{cancel} 2a-3(-x)-3(-5a)-3(7) \\\\= 2a+3x+15a-21 \\\\= (2a+15a)+3x-21 \\\\= 17a+3x-21 .\end{array}