Answer
$(4x^{2}+9)(4x^{2}-9)$
Work Step by Step
If $a$ and $b$ are real numbers, we know that $a^{2}-b^{2}=(a+b)(a-b)$. In words, the difference of the squares of two terms factors as a product of a sum and a difference of those terms.
Therefore, we know that $16x^{4}-81=(4x^{2}+9)(4x^{2}-9)$. In this case, $a=4x^{2}$ and $b=9$.
