\[\frac{\sqrt{6} - \sqrt{5}}{\sqrt{42} - \sqrt{35} - \sqrt{18} + \sqrt{15}} =\]
\[= \frac{\sqrt{6} - \sqrt{5}}{\sqrt{7}\left( \sqrt{6} - \sqrt{5} \right) - \sqrt{3}\left( \sqrt{6} - \sqrt{5} \right)} =\]