Answer
$(x+3)(x-7)$
Work Step by Step
$x^2-4x-21$ has no common monomial factor
$=(\ \ \ \ \ \ \ \ )(\ \ \ \ \ \ \ \ )$
$=(x\ \ \ \ \ \ )(x\ \ \ \ \ \ )$
$=(x+\ )(x-\ )$ The factors of -21 must have unlike signs.
$=(x+3)(x-7)$ 3 and -7 have a sum of -4 and a product of -21.
