Answer
$81a^4-1$
Work Step by Step
Use the formula $(a+b)(a-b)$ to multiply the last 2 factors to obtain:
$=(9a^2+1)[(3a)^2-1^2]
\\=(9a^2+1)(9a^2-1)$
Use the same formula again with $a=9a^2$ and $b=1$ to obtain:
$=(9a^2)^2-1^2
\\=81a^4-1$