18. \((4\sqrt{3}-\sqrt{27}+\sqrt{12})\cdot\sqrt{3}\)

Упростим корни: \(\sqrt{27}=3\sqrt{3}\), \(\sqrt{12}=2\sqrt{3}\).

\((4\sqrt{3}-3\sqrt{3}+2\sqrt{3})\cdot\sqrt{3}=3\sqrt{3}\cdot\sqrt{3}=3\cdot3=9\).

Ответ: 9.