Answer
$(x^{2}-6)(x^{2}+1)$
Work Step by Step
Substituting $x^{2}=t$, the trinomial becomes quadratic,
$t^{3}-5t-6=$
... Searching for two factors of $-6$ whose sum is $-5,$
we find$\qquad-6$ and $+1$
$=(t-6)(t+1)$
... bring back x
= $(x^{2}-6)(x^{2}+1)$