Answer
$5a^6(a-5)(a+1)$
Work Step by Step
Factoring the $GCF=
5a^6
$, the given expression, $
5a^8-20a^7-25a^6
,$ is equivalent to
\begin{array}{l}
5a^6(a^2-4a-5)
.\end{array}
The 2 numbers whose product is $ac=
1(-5)=-5
$ and whose sum is $b=
-4
$ are $\left\{
-5,1
\right\}.$ Using these two numbers to decompose the middle term of the above trinomial, then
\begin{array}{l}
5a^6(a^2-5a+1a-5)
\\\\=
5a^6[(a^2-5a)+(1a-5)]
\\\\=
5a^6[a(a-5)+(a-5)]
\\\\=
5a^6[(a-5)(a+1)]
\\\\=
5a^6(a-5)(a+1)
.\end{array}