Answer
$x^2-9x-90$
Work Step by Step
Using $(a+b)(c+d)=ac+(ad+bc)+bd$, or the FOIL Method, the given expression, $
(x-15)(x+6)
,$ is equivalent to
\begin{array}{l}\require{cancel}
x^2+(6x-15x)-90
\\\\=
x^2-9x-90
.\end{array}
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