#### Answer

$(x+3)(x-7)$

#### Work Step by Step

The leading coefficient is 1.
Thus, to factor the trinomial, look for the factors of the constant term whose sum is equal to the coefficient of the middle term.
Note that:
$-21=3(-7)$ and that $3+(-7)=-4$
Thus, the factors we are looking for are $3$ and $-7$.
This means that factors of the given trinomial are:
$=(x+3)(x-7)$
Check:
$(x+3)(x-7)
\\=x(x-7)+3(x-7)
\\=x^2-7x+3x-21
\\=x^2-4x-21$