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

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

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

We multiply parentheses ..

(+5x^2+20x-15x-60)=0

We get rid of parentheses

5x^2+20x-15x-60=0

We add all the numbers together, and all the variables

5x^2+5x-60=0

a = 5; b = 5; c = -60;
Δ = b2-4ac
Δ = 52-4·5·(-60)
Δ = 1225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1225}=35$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(5)-35}{2*5}=\frac{-40}{10}
=-4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(5)+35}{2*5}=\frac{30}{10}
=3
$