Answer
$(x^2-6)(x^2+1)$
Work Step by Step
The two numbers whose product is $ac=
1(-6)=-6
$ and whose sum is $b=
-5
$ are $\{
-6,1
\}$. Using these two numbers to decompose the middle term, then the factored form of the expression, $
x^4-5x^2-6
$, is
\begin{array}{l}\require{cancel}
x^4-6x^2+x^2-6
\\\\=
(x^4-6x^2)+(x^2-6)
\\\\=
x^2(x^2-6)+(x^2-6)
\\\\=
(x^2-6)(x^2+1)
.\end{array}
