(x+15)(x+20)=0

Solution for (x+15)(x+20)=0 equation:

(x+15)(x+20)=0

We multiply parentheses ..

(+x^2+20x+15x+300)=0

We get rid of parentheses

x^2+20x+15x+300=0

We add all the numbers together, and all the variables

x^2+35x+300=0

a = 1; b = 35; c = +300;
Δ = b2-4ac
Δ = 352-4·1·300
Δ = 25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{25}=5$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(35)-5}{2*1}=\frac{-40}{2}
=-20
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(35)+5}{2*1}=\frac{-30}{2}
=-15
$