Chapter 4 - Quadratic Functions and Equations - 4-4 Factoring Quadratic Expressions - Lesson Check - Page 221: 4

$(5y+6)(5y-6)$

Using the factoring of the difference of $2$ squares which is given by $a^2-b^2=(a+b)(a-b),$ the given $\text{ expression, }$ $ 25y^2-36 ,$ is equivalent to \begin{align*} & (5y)^2-(6)^2 \\&= (5y+6)(5y-6) \end{align*}