(3x-9)(5x+20)=0

Solution for (3x-9)(5x+20)=0 equation:

(3x-9)(5x+20)=0

We multiply parentheses ..

(+15x^2+60x-45x-180)=0

We get rid of parentheses

15x^2+60x-45x-180=0

We add all the numbers together, and all the variables

15x^2+15x-180=0

a = 15; b = 15; c = -180;
Δ = b2-4ac
Δ = 152-4·15·(-180)
Δ = 11025
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{11025}=105$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-105}{2*15}=\frac{-120}{30}
=-4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+105}{2*15}=\frac{90}{30}
=3
$