\[\left| \sqrt{24} - 3\sqrt{3} \right| = \left| \sqrt{24} - \sqrt{27} \right| =\]
\[= - \left( \sqrt{24} - \sqrt{27} \right) =\]
\[= \sqrt{27} - \sqrt{24} =\]
\[= 3\sqrt{3} - 2\sqrt{6}\]
\[\sqrt{24} - \sqrt{27} < 0.\]