19. \((\sqrt{50}-\sqrt{8})\cdot\sqrt{2}\)

Упростим корни: \(\sqrt{50}=5\sqrt{2}\), \(\sqrt{8}=2\sqrt{2}\).

\((5\sqrt{2}-2\sqrt{2})\cdot\sqrt{2}=3\sqrt{2}\cdot\sqrt{2}=3\cdot2=6\).

Ответ: 6.