Chapter 1 - Introduction to Algebraic Expressions - 1.8 Exponential Notation and Order of Operations - 1.8 Exercise Set - Page 67: 87

$21t-r$

Using $a(b+c)=ab+ac$ or the Distributive Property and then combining like terms, the given expression, $ 9t-7r+2(3r+6t) $, simplifies to \begin{array}{l}\require{cancel} 9t-7r+2(3r)+2(6t) \\\\= 9t-7r+6r+12t \\\\= (9t+12t)+(-7r+6r) \\\\= 21t-r .\end{array}