(3x-9)(4x+20)=0

Solution for (3x-9)(4x+20)=0 equation:

(3x-9)(4x+20)=0

We multiply parentheses ..

(+12x^2+60x-36x-180)=0

We get rid of parentheses

12x^2+60x-36x-180=0

We add all the numbers together, and all the variables

12x^2+24x-180=0

a = 12; b = 24; c = -180;
Δ = b2-4ac
Δ = 242-4·12·(-180)
Δ = 9216
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{9216}=96$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(24)-96}{2*12}=\frac{-120}{24}
=-5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(24)+96}{2*12}=\frac{72}{24}
=3
$