Answer
$k=12$.
Work Step by Step
The factor is $x+3$.
$x^2-x-k$.
Replace the value of $x$ with $-3$ and set equal to zero.
$\Rightarrow (-3)^2-(-3)-k=0$
Clear the parentheses.
$\Rightarrow 9+3-k=0$
Simplify.
$\Rightarrow 12-k=0$
Add $k$ to both sides.
$\Rightarrow 12-k+k=0+k$
Simplify.
$\Rightarrow 12=k$.
