#### Answer

$x-c$

#### Work Step by Step

The Factor Theorem$:$
Let $f(x)$ be a polynomial.
$\mathrm{a}$. If $f(c)=0$, then $\ \ x-c\ \ $ is a factor of $f(x)$.
$\mathrm{b}$. If $\ \ x-c\ \ $ is a factor of $f(x)$, then $f(c)=0$.
Case a: fill in the blank with
$x-c$