\[\frac{6 - \sqrt{35}}{6 + \sqrt{35}} = 71 - 12\sqrt{35}\]
\[\frac{\left( 6 - \sqrt{35} \right)\left( 6 - \sqrt{35} \right)}{\left( 6 + \sqrt{35} \right)\left( 6 - \sqrt{35} \right)} =\]
\[= \frac{\left( 6 - \sqrt{35} \right)^{2}}{6^{2} - \left( \sqrt{35} \right)^{2}} =\]
\[= \frac{36 - 12\sqrt{35} + 35}{36 - 35} =\]
\[= 71 - 12\sqrt{35} \Longrightarrow ч.т.д.\]