Answer
$81x^4-1$
Work Step by Step
First we apply the Sum and Difference of Two Terms for the second and the third factor:
$(9x^2+1)[(3x+1)(3x-1)]$
$=(9x^2+1)[(3x)^2-1^2]$
$=(9x^2+1)(9x^2-1).$
Then we apply the Sum and Difference of Two Terms once again:
$(9x^2+1)(9x^2-1)$
$=(9x^2)^2-1^2$
$=81x^4-1.$
