## Intermediate Algebra (6th Edition)

$x=45$
Using the properties of equality, the given equation, $-\sqrt{3x+9}=-12 ,$ is equivalent to \begin{array}{l}\require{cancel} \sqrt{3x+9}=12 .\end{array} Squaring both sides of the equation above, then the solution/s is/are \begin{array}{l}\require{cancel} 3x+9=144 \\\\ 3x=144-9 \\\\ 3x=135 \\\\ x=45 .\end{array} Upon checking, only $x=45$ satisfies the original equation.