Answer
$(x^{2}-5)(x^{2}+1)$
Work Step by Step
Substituting $x^{2}=t$, the trinomial becomes quadratic,
$t^{3}-4t-5=$
... Searching for two factors of $-5$ whose sum is $-4,$
we find$\qquad-5$ and $+1$
$=(t-5)(t+1)$
... bring back x
= $(x^{2}-5)(x^{2}+1)$