Chapter R - Elementary Algebra Review - R.2 Equations, Inequalities, and Problem Solving - R.2 Exercise Set - Page 946: 26

$x=12$

Removing the grouping symbols and combining like terms, the value of the variable that satisfies the given expression, $ 18=9-(3-x) ,$ is \begin{array}{l}\require{cancel} 18=9-3+x \\\\ -x=9-3-18 \\\\ -x=-12 \\\\ x=12 .\end{array}