#### Answer

$\color{blue}{(9y+\frac{1}{7})(9y-\frac{1}{7})}$

#### Work Step by Step

With $81y^2=(9y)^2$ and $\frac{1}{49}=(\frac{1}{7})^2$, the given binomial is equivalent to:
$=(9y)^2-(\frac{1}{7})^2$
Factor using the formula $a^2-b^2=(a+b)(a-b)$ with $a=9y$ and $b=\frac{1}{7}$ to obtain:
$=\color{blue}{(9y+\frac{1}{7})(9y-\frac{1}{7})}$