Chapter 5 - Exponents and Polynomials - 5.2 - Multiplying Monomials - Problem Set 5.2 - Page 200: 55

$-18a^3+30a^2+42a$

Using $a(b+c)=ab+ac$ (or the Distributive Property), the given expression, $ -6a(3a^2-5a-7) ,$ is equivalent to \begin{array}{l}\require{cancel} -6a(3a^2)-6a(-5a)-6a(-7) \\\\= -18a^3+30a^2+42a .\end{array}