Answer
$81x^4-256$
Work Step by Step
$$(3x+4)(3x-4)(9x^2+16)$$ To multiply the sum of two terms by the difference of two terms, use the formula: $(a+b)(a-b)=a^2-b^2$: $$=[(3x)^2-(4)^2](9x^2+16)$$ $$=(9x^2-16)(9x^2+16)$$ Another product of the sum and difference of two terms! $$=(9x^2)^2-(16)^2$$ $$=81x^4-256$$
