Chapter 7 - Section 7.4 - Adding, Subtracting, and Multiplying Radical Expressions - Exercise Set - Page 439: 28

$6xy\sqrt{2y}-8xy\sqrt{2y}$

Using the properties of radicals, the given expression, $ 3\sqrt{8x^2y^3}-2x\sqrt{32y^3} ,$ simplifies to \begin{array}{l}\require{cancel} 3\sqrt{4x^2y^2\cdot2y}-2x\sqrt{16y^2\cdot2y} \\\\= 3\sqrt{(2xy)^2\cdot2y}-2x\sqrt{(4y)^2\cdot2y} \\\\= 3\cdot2xy\sqrt{2y}-2x\cdot4y\sqrt{2y} \\\\= 6xy\sqrt{2y}-8xy\sqrt{2y} .\end{array}