Chapter 7 - Section 7.4 - Adding, Subtracting, and Multiplying Radical Expressions - Exercise Set - Page 439: 30

$16xy\sqrt[3]{3y}$

Using the properties of radicals, then, \begin{array}{l} 2\sqrt[3]{24x^3y^4}+4x\sqrt[3]{81y^4} \\= 2\sqrt[3]{8x^3y^3\cdot3y}+4x\sqrt[3]{27y^3\cdot3y} \\= 2\cdot2xy\sqrt[3]{3y}+4x\cdot3y\sqrt[3]{3y} \\= 4xy\sqrt[3]{3y}+12xy\sqrt[3]{3y} \\= 16xy\sqrt[3]{3y} .\end{array}