[tex]\bf \textit{difference of squares}
\\ \quad \\
(a-b)(a+b) = a^2-b^2\qquad \qquad
a^2-b^2 = (a-b)(a+b)\\\\
-------------------------------\\\\
81y^2-4\quad
\begin{cases}
81=9^2\\
4=2^2
\end{cases}\implies 9^2y^2-2^2\implies (9y)^2-2^2
\\\\\\
(9y-2)(9y+2)[/tex]
