Chapter 1 - Introduction to Algebraic Expressions - 1.8 Exponential Notation and Order of Operations - 1.8 Exercise Set - Page 67: 88

$10m-12n$

Using $a(b+c)=ab+ac$ or the Distributive Property and then combining like terms, the given expression, $ 4m-9n+3(2m-n) $, simplifies to \begin{array}{l}\require{cancel} 4m-9n+3(2m)+3(-n) \\\\= 4m-9n+6m-3n \\\\= (4m+6m)+(-9n-3n) \\\\= 10m-12n .\end{array}