Answer
$20a^2-9a-20$
Work Step by Step
Using $(a+b)(c+d)=ac+(ad+bc)+bd$, or the FOIL Method, the given expression, $
(5a+4)(4a-5)
,$ is equivalent to
\begin{array}{l}\require{cancel}
20a^2+(-25a+16a)-20
\\\\=
20a^2-9a-20
.\end{array}
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