Chapter 7 - Section 7.4 - Adding, Subtracting, and Multiplying Radical Expressions - Exercise Set - Page 439: 51

$3\sqrt{x}-x\sqrt{3}$

Using the Distributive Property and the properties of radicals, then, \begin{array}{l} \sqrt{3x}\left( \sqrt{3}-\sqrt{x} \right) \\= \sqrt{9x}-\sqrt{3x^2} \\= \sqrt{9\cdot x}-\sqrt{x^2\cdot3} \\= 3\sqrt{x}-x\sqrt{3} .\end{array}