Answer
$5a^{2}(2a-3)(2a+3)$
Work Step by Step
$ 20a^{4}-45a^{2}\qquad$...factor out the common term, $5a^{2}$
$=5a^{2}(4a^{2}-9)\qquad$...recognize the difference of two squares: $a^{2}-b^{2}=(a-b)(a+b)$
$=5a^{2}(2a-3)(2a+3)$
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