Chapter 7 - Section 7.4 - Adding, Subtracting, and Multiplying Radical Expressions - Exercise Set - Page 438: 20

$-4\sqrt[3]{x}+7\sqrt{7}$

Using the properties of radicals, then, \begin{array}{l} 3\sqrt{7}-\sqrt[3]{x}+4\sqrt{7}-3\sqrt[3]{x} \\= (-\sqrt[3]{x}-3\sqrt[3]{x})+(3\sqrt{7}+4\sqrt{7}) \\= -4\sqrt[3]{x}+7\sqrt{7} .\end{array}