\(\begin{array}{l}
\left( {\dfrac{1}{{\sqrt x - 3}} - \dfrac{1}{{\sqrt x + 3}}} \right):\dfrac{3}{{\sqrt x - 3}}\\
= \dfrac{{\sqrt x + 3 - \sqrt x + 3}}{{\left( {\sqrt x - 3} \right)\left( {\sqrt x + 3} \right)}}.\dfrac{{\sqrt x - 3}}{3}\\
= \dfrac{6}{{\left( {\sqrt x - 3} \right)\left( {\sqrt x + 3} \right)}}.\dfrac{{\sqrt x - 3}}{3}\\
= \dfrac{2}{{\sqrt x + 3}}
\end{array}\)
