Answer
$(x-4)(x+3)$
Work Step by Step
The two numbers whose product is $ac=
1(-12)=-12
$ and whose sum is $b=
-1
$ are $\{
-4,3
\}$. Using these two numbers to decompose the middle term, then the factored form of the expression, $
x^2-x-12
$, is
\begin{array}{l}\require{cancel}
x^2-4x+3x-12
\\\\=
(x^2-4x)+(3x-12)
\\\\=
x(x-4)+3(x-4)
\\\\=
(x-4)(x+3)
.\end{array}
