\[\left( \sqrt{6} + 4\sqrt{3} \right)\left( \sqrt{6} - 4\sqrt{3} \right) =\]
\[= \left( \sqrt{6} \right)^{2} - \left( 4\sqrt{3} \right)^{2} = 6 - 16 \cdot 3 =\]
\[= 6 - 48 = - 42\]