(5x-15)(4x+28)=0

Solution for (5x-15)(4x+28)=0 equation:

(5x-15)(4x+28)=0

We multiply parentheses ..

(+20x^2+140x-60x-420)=0

We get rid of parentheses

20x^2+140x-60x-420=0

We add all the numbers together, and all the variables

20x^2+80x-420=0

a = 20; b = 80; c = -420;
Δ = b2-4ac
Δ = 802-4·20·(-420)
Δ = 40000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{40000}=200$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(80)-200}{2*20}=\frac{-280}{40}
=-7
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(80)+200}{2*20}=\frac{120}{40}
=3
$