\[y = f(x);\ \ f(x) = \sqrt{x};\ \ \ f(x + 1)\]
\[= \frac{\sqrt{7} + \sqrt{3} - \sqrt{7} + \sqrt{3}}{\left( \sqrt{7} - \sqrt{3} \right)\left( \sqrt{7} + \sqrt{3} \right)} \cdot \sqrt{300} =\]
\[= \frac{2\sqrt{3}}{4} \cdot 10\sqrt{3} = 15\]
\[f(15 + 1) = f(16) = \sqrt{16} = 4\]
\[Ответ:4.\ \]