#### Answer

$(x^2-2)(x^2-3)$

#### Work Step by Step

The two numbers whose product is $ac=
1(6)=6
$ and whose sum is $b=
-5
$ are $\{
-2,-3
\}$. 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^4-2x^2-3x^2+6
\\\\=
(x^4-2x^2)-(3x^2-6)
\\\\=
x^2(x^2-2)-3(x^2-2)
\\\\=
(x^2-2)(x^2-3)
.\end{array}