#### Answer

(a) $6\sqrt[3]{3}$
(b) $\frac{2\sqrt{3}}{5}$
(c) $\frac{3\sqrt{2}}{7}$

#### Work Step by Step

(a) $2\sqrt[3]{81}=2\sqrt[3]{3*3^{3}}=2*3\sqrt[3]{3}=6\sqrt[3]{3}$
(b) $\frac{\sqrt{12}}{\sqrt{25}}=\frac{\sqrt{3*2^{2}}}{5}=\frac{2\sqrt{3}}{5}$
(c)$\sqrt{\frac{18}{49}}=\sqrt{\frac{2*3^{2}}{7^{2}}}=\frac{3\sqrt{2}}{7}$