Chapter 5 - Section 5.4 - Multiplying Polynomials - Exercise Set - Page 289: 36

$9x^2y^2-4b^2$

Using the special product of the sum and difference of like terms, $(a+b)(a-b)=a^2-b^2$, the given expression is equivalent to \begin{align*} & (3xy-2b)(3xy+2b) \\\\& (3xy)^2-(2b)^2 \\\\& 9x^2y^2-4b^2 .\end{align*}