\[5\sqrt{6} - \sqrt{216} + 3\sqrt{24} =\]
\[= 5\sqrt{6} - \sqrt{36 \cdot 6} + 3\sqrt{4 \cdot 6} =\]
\[= 5\sqrt{6} - 6\sqrt{6} + 3 \cdot 2\sqrt{6} = 5\sqrt{6}.\]