\[y = f(x);\ \ \ \ f(x) = \sqrt{x};\ \ \ \ \ \]
\[f(x + 6)\]
\[x = \left( \frac{1}{3 - \sqrt{5}} - \frac{1}{3 + \sqrt{5}} \right) \cdot \sqrt{80} =\]
\[= \frac{3 + \sqrt{5} - 3 + \sqrt{5}}{\left( 3 - \sqrt{5} \right)\left( 3 + \sqrt{5} \right)} \cdot \sqrt{80} =\]
\[= \frac{2\sqrt{5}}{4} \cdot 4\sqrt{5} = 10\]
\[f(10 + 6) = f(16) = \sqrt{16} = 4\]
\[Ответ:\ \ 4.\]