Answer
$a^2-1.2a+0.32$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(a-0.4)(a-0.8)
$, is equivalent to
\begin{array}{l}\require{cancel}
a(a)+a(-0.8)-0.4(a)-0.4(-0.8)
\\\\=
a^2-0.8a-0.4a+0.32
\\\\=
a^2-1.2a+0.32
.\end{array}