Answer
$a^3-a^2-10a+12$
Work Step by Step
Using $a(b+c)=ab+ac$ or the Distributive Property, the given expression, $
(a-3)(a^2+2a-4)
$, is equivalent to
\begin{array}{l}\require{cancel}
a(a^2)+a(2a)+a(-4)-3(a^2)-3(2a)-3(-4)
\\\\=
a^3+2a^2-4a-3a^2-6a+12
\\\\=
a^3+(2a^2-3a^2)+(-4a-6a)+12
\\\\=
a^3-a^2-10a+12
.\end{array}
