[tex]
\sqrt{15-x}=-3\sqrt{2x+3}\impliedby \textit{divide by -3}
\\ \quad \\
\cfrac{\sqrt{15-x}}{-3}=\sqrt{2x+3}\impliedby \textit{squares both sides}
\\ \quad \\
\left( \cfrac{\sqrt{15-x}}{-3} \right)^2=(\sqrt{2x+3})^2\implies
\cfrac{(\sqrt{15-x})^2}{(-3)^2}=(\sqrt{2x+3})^2
\\ \quad \\
\cfrac{15-x}{9}=2x+3\impliedby \textit{multiply both sides by 9}
\\ \quad \\
15-x=18x+27\impliedby \textit{solve for "x"}[/tex]
