Answer
$(x+1)(3x-28)$
Work Step by Step
Factoring by grouping:
1. Multiply the leading coefficient, a, and the constant, c.
2. Find the factors of ac whose sum is b.
3. Rewrite the middle term, bx, as a sum or difference using the factors from step 2.
4. Factor by grouping
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Always start by searching for a GCF ... (there are none other than 1).
1. $\quad ac=-84$
2. $\quad$sum = $-25 \quad$... factors: $-28$ and $+3$
3. $\quad 3x^{2}-25x-28=(3x^{2}+3x)+(-28x-28)$
4. $\quad$... $=3x(x+1) +(-28)(x+1) =(x+1)(3x-28)$
Check by FOIL
$F:\quad 3x^{2}$
$O:\quad -28x$
$I:\quad +3x$
$L:\quad -28$
$(x+1)(3x-28)$ = $3x^{2}-25x-28$
