б) √74 / √6.√21

б) $$ \frac{\sqrt{74}}{\sqrt{6} \cdot \sqrt{21}} = \frac{\sqrt{74}}{\sqrt{2 \cdot 3} \cdot \sqrt{3 \cdot 7}} = \frac{\sqrt{74}}{\sqrt{2} \cdot 3 \cdot \sqrt{7}} = \frac{\sqrt{74}}{3\sqrt{14}} = \frac{\sqrt{74} \cdot \sqrt{14}}{3 \cdot 14} = \frac{\sqrt{1036}}{42} = \frac{\sqrt{4 \cdot 259}}{42} = \frac{2 \sqrt{259}}{42} = \frac{\sqrt{259}}{21}$$.

Ответ: $$\frac{\sqrt{259}}{21}$$