Answer
$81x^4-256$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
Hence here: $(3x+4)(3x-4)(9x^2+16)=((3x)^2-4^2)(9x^2+16)=(9x^2-16)(9x^2+16)=(9x^2)^2-16^2=81x^4-256$
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