Chapter 3 - Equations and Problem Solving - 3.3 - More on Solving Equations and Problem Solving - Problem Set 3.3 - Page 114: 2

$x=3$

Using the properties of equality and combining like terms, the value of the variable that satisfies the given equation, $ 3x+9+4x=30 ,$ is \begin{array}{l}\require{cancel} 3x+4x=30-9 \\\\ 7x=21 \\\\ x=\dfrac{21}{7} \\\\ x=3 .\end{array}