Chapter 7 - Review - Page 469: 98

$4x-9y$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the expression $ (2\sqrt{x}-3\sqrt{y})(2\sqrt{x}+3\sqrt{y}) $ simplifies to \begin{array}{l} (2\sqrt{x})^2-(3\sqrt{y})^2 \\= 4x-9y .\end{array}