\[\left( 6\sqrt{m} + 8\sqrt{n} \right)\left( 6\sqrt{m} - 8\sqrt{n} \right) =\]
\[= \left( 6\sqrt{m} \right)^{2} - \left( 8\sqrt{n} \right)^{2} =\]
\[= 36m - 64n\]