13.3. \((\sqrt{18}-\sqrt{2})\cdot\sqrt{2}\)

\(\sqrt{18}=3\sqrt{2}\).

\((\sqrt{18}-\sqrt{2})\cdot\sqrt{2}=(3\sqrt{2}-\sqrt{2})\cdot\sqrt{2}=2\sqrt{2}\cdot\sqrt{2}=2\cdot2=4\).

Ответ: \(4\)