Answer
$(x-5)(x+4)$
Work Step by Step
The given expression is equivalent to
\begin{array}{l}\require{cancel}
x^2-20-x
\\\\=
x^2-x-20
.\end{array}
The trinomial expression above in the form $ax^2+bx+c,$ has $c$ equal to $
-20
$ and $b$ equal to $
-1
.$ The two numbers whose product is $c$ and whose sum is $b$ are $\{
-5,4
\}.$ Hence, the factored form of the expression above is
\begin{array}{l}\require{cancel}
(x-5)(x+4)
.\end{array}
