$\text{I.}\sqrt{7}\cdot \sqrt{7}=\sqrt{7\cdot 3}=\sqrt{21}$

$\text{II.}\sqrt{3}\cdot \sqrt{18}=\sqrt{3\cdot 18}=\sqrt{54}$ $\text{II.}\sqrt{3}\cdot \sqrt{18}=\sqrt{3\cdot 18}=\sqrt{54}$

$\text{III.}\sqrt{10}\cdot \sqrt{20}=\sqrt{10\cdot 20}=\sqrt{200}$

$\text{IV.}\sqrt{3}\cdot \sqrt{75}=\sqrt{3\cdot 75}=\sqrt{225}$

$\text{V.}\sqrt{4}\cdot \sqrt{0,25}=\sqrt{4\cdot 0,25}=\sqrt{1}$

$\text{VI.}\sqrt[3]{36}\cdot \sqrt[3]{6}=\sqrt[3]{36\cdot 6}=\sqrt[3]{216}$