Answer
$9x^4 - 16x^2$
Work Step by Step
RECALL:
$(a+b)(a-b)=a^2-b^2$
Use the formula above with $a=3x^2$ and $b=4x$ to obtain:
$=(3x^2)^2-(4x)^2
\\=9x^4 - 16x^2$
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