Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 376: 53

$9x^4-y^2$

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