8) √7 ⋅ 12 ⋅ √21 =

8) $$\sqrt{7} \cdot 12 \cdot \sqrt{21} = \sqrt{7} \cdot \sqrt{21} \cdot 12 = \sqrt{7 \cdot 21} \cdot 12 = \sqrt{7 \cdot 7 \cdot 3} \cdot 12 = 7 \sqrt{3} \cdot 12 = 84 \sqrt{3}$$

Ответ: $$84\sqrt{3}$$