Chapter 5 - Section 5.4 - Multiplying Polynomials - Exercise Set - Page 289: 31

$36y^2-1$

Using the special product on multiplying the sum and difference of like terms, $(a+b)(a-b)=a^2-b^2$, the given expression simplifies to \begin{align*} & (6y-1)(6y+1) \\\\&= (6y)^2-1^2 \\\\&= 36y^2-1 .\end{align*}