(-3x-9)(4x+28)=0

Solution for (-3x-9)(4x+28)=0 equation:

(-3x-9)(4x+28)=0

We multiply parentheses ..

(-12x^2-84x-36x-252)=0

We get rid of parentheses

-12x^2-84x-36x-252=0

We add all the numbers together, and all the variables

-12x^2-120x-252=0

a = -12; b = -120; c = -252;
Δ = b2-4ac
Δ = -1202-4·(-12)·(-252)
Δ = 2304
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{2304}=48$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-120)-48}{2*-12}=\frac{72}{-24}
=-3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-120)+48}{2*-12}=\frac{168}{-24}
=-7
$