Answer
$16x^4 - 25x^2$
Work Step by Step
RECALL:
$(a+b)(a-b)=a^2-b^2$
Use the formula above with $a=4x^2$ and $b=5x$ to obtain:
$=(4x^2)^2-(5x)^2
\\=16x^4 - 25x^2$
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