Answer
$\color{blue}{(3a+4)(3a-4)}$
Work Step by Step
Note that $9a^2=(3a)^2$ and $16=4^2$.
Thus, the given binomial is equivalent to:
$=(3a)^2-4^2$
Factor using the formula $a^2-b^2=(a+b)(a-b)$ where $a=3a$ and $b=4$ to obtain:
$=\color{blue}{(3a+4)(3a-4)}$
