Chapter 7 - Section 7.5 - Rationalizing Denominators and Numerators of Radical Expressions - Exercise Set - Page 445: 17

$\dfrac{2\sqrt[]{21}}{7}$

Rationalizing the denominator of $ \dfrac{2\sqrt{3}}{\sqrt[]{7}} $ results to \begin{array}{l} \dfrac{2\sqrt{3}}{\sqrt[]{7}} \cdot \dfrac{\sqrt[]{7}}{\sqrt[]{7}} \\\\= \dfrac{2\sqrt[]{21}}{\sqrt[]{49}} \\\\= \dfrac{2\sqrt[]{21}}{7} .\end{array}