Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 368: 68

$16y^2-81$

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, $ (4y+9)(4y-9) ,$ is \begin{array}{l}\require{cancel} 4y(4y)-9(9) \\\\= 16y^2-81 .\end{array}