$\text{a)}$  

$\sqrt{3\cdot 4\cdot 12}=\sqrt{12\cdot 12}=12$  

$\sqrt{2\cdot 9\cdot 18}=\sqrt{18\cdot 18}=18$  

$\sqrt{7\cdot 3\cdot 21}=\sqrt{21\cdot 21}=21$  

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$\text{b)}$  

$\sqrt{2\cdot 6\cdot 3}=\sqrt{6\cdot 6}=6$  

$\sqrt{2\cdot 18\cdot 9}=\sqrt{18\cdot 18}=18$  

$\sqrt{2\cdot 8\cdot 16}=\sqrt{16\cdot 16}=16$  

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$\text{c)}\sqrt{25\cdot 18\cdot 2}=\sqrt{25\cdot 36}=\sqrt{25}\cdot \sqrt{36}=5\cdot 6=30$  

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$\text{d)}\sqrt{2\cdot 25\cdot 8}=\sqrt{25\cdot 16}=\sqrt{25}\cdot \sqrt{16}=5\cdot 4=20$  

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$\text{e)}\sqrt{3\cdot 9\cdot 12}=\sqrt{9\cdot 36}=\sqrt{9}\cdot \sqrt{36}=3\cdot 6=18$  

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$\text{f)}\sqrt{6\cdot 12\cdot 8}=\sqrt{6\cdot 4\cdot 3\cdot 8}=\sqrt{24\cdot 24}=24$  

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$\text{g)}\sqrt[3]{3\cdot 9\cdot 64}=\sqrt[3]{27\cdot 64}=\sqrt[3]{27}\cdot \sqrt[3]{64}=3\cdot 4=12$  

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$\text{h)}\sqrt[3]{36\cdot 30\cdot 25}=\sqrt[3]{36\cdot 6\cdot 5\cdot 25}=\sqrt[3]{216\cdot 125}=\sqrt[3]{216}\cdot \sqrt[3]{125}=6\cdot 5=30$