\[\left( 3\sqrt{7} - \sqrt{5} \right)\left( 3\sqrt{7} + \sqrt{5} \right) =\]
\[= \left( 3\sqrt{7} \right)^{2} - \left( \sqrt{5} \right)^{2} = 9 \cdot 7 - 5 =\]
\[= 63 - 5 = 58\]