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Вот ваше решение:

$\frac{3-\sqrt{3} }{\sqrt{21}-\sqrt{7}} = \frac{(3-\sqrt{3})(\sqrt{21}+\sqrt{7})}{(\sqrt{21}-\sqrt{7})(\sqrt{21}+\sqrt{7})} = \frac{3\sqrt{21}+3\sqrt{7}-\sqrt{63}-\sqrt{21}}{21-7} =\\\\\frac{3\sqrt{21}+3\sqrt{7}-3\sqrt{7}-\sqrt{21}}{14}} = \frac{3\sqrt{21}-\sqrt{21}}{14} = \\\\\frac{2\sqrt{21}}{14} = \frac{\sqrt{21}}{7}$