Answer
$(3x-1)(3x+1)(9x^2+1)$
Work Step by Step
The given expression can be written as
$(9x^2)^2-1^2.$
The expression above is a difference of two squares.
RECALL:
$a^2-b^2=(a-b)(a+b).$
Factor the difference of two squares using the formula above with $a=9x^2$ and $b=1$ to obtain
$(9x^2-1)(9x^2+1)
\\=[(3x)^2-1^2](9x^2+1).$
Factor the difference of two squares with $a=3x$ and $b=1$ to obtain
$(3x-1)(3x+1)(9x^2+1).$
