Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 26

$y^2-25$

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+5)(y-5) ,$ is \begin{array}{l}\require{cancel} y(y)-5(5) \\\\= y^2-25 .\end{array}