a) $$5\sqrt{3} = \sqrt{5^2\times3} = \sqrt{25\times3} = \sqrt{75}$$
$$\textbf{Ответ: } \sqrt{75}$$
б) $$x\sqrt{2} = \sqrt{x^2\times2} = \sqrt{2x^2}$$
$$\textbf{Ответ: } \sqrt{2x^2}$$
в) $$y\sqrt{\frac{2}{y}} = \sqrt{y^2 \cdot \frac{2}{y}} = \sqrt{2y}$$
$$\textbf{Ответ: } \sqrt{2y}$$