(5x-15)(4x+8)=0

Solution for (5x-15)(4x+8)=0 equation:

(5x-15)(4x+8)=0

We multiply parentheses ..

(+20x^2+40x-60x-120)=0

We get rid of parentheses

20x^2+40x-60x-120=0

We add all the numbers together, and all the variables

20x^2-20x-120=0

a = 20; b = -20; c = -120;
Δ = b2-4ac
Δ = -202-4·20·(-120)
Δ = 10000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{10000}=100$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-100}{2*20}=\frac{-80}{40}
=-2
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+100}{2*20}=\frac{120}{40}
=3
$