## Elementary Algebra

$x=3$
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}