Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 38

$y^4-4$

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