Answer
$4a^2(3a+1)(3a-1)$.
Work Step by Step
The given polynomial is
$= 36a^4-4a^2$
Factor out $4a^2$.
$= 4a^2(9a^2-1)$
Write the the polynomial as $a^2-b^2$.
$= 4a^2((3a)^2-1^2)$
Use difference of two square pattern
$a^2-b^2=(a+b)(a-b)$.
We have $a=3a$ and $b=1$.
$= 4a^2(3a+1)(3a-1)$
Hence, the factor of the polynomial is $4a^2(3a+1)(3a-1)$.