14. \(2\sqrt{8m}+\sqrt{18m}-\sqrt{50m}\)

Упростим каждый корень:

\(\sqrt{8m}=2\sqrt{2m}\), \(\sqrt{18m}=3\sqrt{2m}\), \(\sqrt{50m}=5\sqrt{2m}\).

Подставим:

\(2\cdot2\sqrt{2m}+3\sqrt{2m}-5\sqrt{2m}=4\sqrt{2m}+3\sqrt{2m}-5\sqrt{2m}=2\sqrt{2m}\).

Ответ: \(2\sqrt{2m}\)