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