\(\begin{array}{l}
a)\,DK:\left\{ \begin{array}{l}
x \ge 0\\
\sqrt x - 3 \ne 0\\
x \ne 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
x > 0\\
x \ne 9
\end{array} \right.\\
b)\,P = \frac{{\sqrt x + 3 + \sqrt x - 3}}{{\left( {\sqrt x + 3} \right)\left( {\sqrt x - 3} \right)}}.\frac{{x - 9}}{{4x}}\\
= \frac{{2\sqrt x }}{{x - 9}}.\frac{{x - 9}}{{4x}} = \frac{1}{{2\sqrt x }}\\
c)\,P = 3\\
\Leftrightarrow \frac{1}{{2\sqrt x }} = 3\\
\Rightarrow 6\sqrt x = 1\\
\Leftrightarrow \sqrt x = \frac{1}{6}\\
\Leftrightarrow x = \frac{1}{{36}}\,\left( {tm} \right)
\end{array}\)
