let's say she's "x" years old.
[tex]\bf \stackrel{her~age}{\cfrac{3x}{4}+9}~~=~~\stackrel{her~age}{\cfrac{x}{4}+21}\impliedby \textit{now, let's multiply both sides by }\stackrel{LCD}{4}
\\\\\\
\boxed{4}\cdot \cfrac{3x}{4}+\boxed{4}\cdot 9~~=~~\boxed{4}\cdot \cfrac{x}{4}+\boxed{4}\cdot 21\implies 3x+36=x+84
\\\\\\
2x=48\implies x=\cfrac{48}{2}\implies x=24[/tex]
