Answer
$(7x+1)(3x-4)$
Work Step by Step
$21x^{2}-25x-4$
Two factors of $ac=-84$, whose sum is $b=-25$
are $3$ and $-28$.
Rewrite the middle term and factor in pairs.
$21x^{2}-25x-4$
$=21x^{2}+3x-28x-4$
$=3x(7x+1)-4(7x+1)]\qquad$...factor out the common binomial factor,$7x+1$.
$=(7x+1)(3x-4)$
