\[\sqrt{26 - 6\sqrt{17}} - \sqrt{21 - 4\sqrt{17}} =\]
\[= \sqrt{\left( \sqrt{17} - 3 \right)^{2}} - \sqrt{\left( \sqrt{17} - 2 \right)^{2}} =\]
\[= \sqrt{17} - 3 - \left( \sqrt{17} - 2 \right) =\]