\[y = f(x);\ \ \ \ f(x) = \sqrt{x};\ \ \ \ \]
\[f(x - 3)\]
\[= \frac{\sqrt{5} + \sqrt{3} - \sqrt{5} + \sqrt{3}}{\left( \sqrt{5} - \sqrt{3} \right)\left( \sqrt{5} + \sqrt{3} \right)} \cdot \sqrt{48} =\]
\[= \frac{2\sqrt{3}}{2} \cdot 4\sqrt{3} = 12\]
\[f(12 - 3) = f(9) = \sqrt{9} = 3\]
\[Ответ:3.\ \]