Ответ:
\[\frac{\sqrt{7} - \sqrt{6}}{\sqrt{56} - \sqrt{48} - \sqrt{21} + \sqrt{18}} =\]

\[= \frac{\sqrt{7} - \sqrt{6}}{\sqrt{8}\left( \sqrt{7} - \sqrt{6} \right) - \sqrt{3}\left( \sqrt{7} - \sqrt{6} \right)} =\]

\[\frac{\sqrt{7} - \sqrt{6}}{\sqrt{56} - \sqrt{48} - \sqrt{21} + \sqrt{18}} =\]

\[= \frac{\sqrt{7} - \sqrt{6}}{\sqrt{8}\left( \sqrt{7} - \sqrt{6} \right) - \sqrt{3}\left( \sqrt{7} - \sqrt{6} \right)} =\]
