a) $$15\sqrt{2} = \sqrt{15^2 \cdot 2} = \sqrt{225 \cdot 2} = \sqrt{450}$$
б) $$-8\sqrt{3} = -\sqrt{8^2 \cdot 3} = -\sqrt{64 \cdot 3} = -\sqrt{192}$$
Ответ: a) $$\sqrt{450}$$; б) $$-\sqrt{192}$$