ĐK:
$\left\{ \begin{matrix} x \geq 0 \\ x \neq 9 \\ x \neq 4 \end{matrix} \right.$
$
A= (\frac{x-3\sqrt x}{x-9}-1): (\frac{9-x}{x+\sqrt x -6}+\frac{\sqrt x - 3}{\sqrt x - 2} - \frac{\sqrt x - 2}{\sqrt x +3} ) \\
A= \frac{-3\sqrt x+9}{x-9} : \frac{9-x+(\sqrt x - 3)(\sqrt x+3) - (\sqrt x - 2)^2}{(\sqrt x+3)(\sqrt x-2)} \\
A=\frac{-3}{\sqrt x+3}:\frac{-\sqrt x+2}{\sqrt x +3} = \frac{3}{\sqrt x-2}
$