(s+4)(s-15)=0

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

(s+4)(s-15)=0

We multiply parentheses ..

(+s^2-15s+4s-60)=0

We get rid of parentheses

s^2-15s+4s-60=0

We add all the numbers together, and all the variables

s^2-11s-60=0

a = 1; b = -11; c = -60;
Δ = b2-4ac
Δ = -112-4·1·(-60)
Δ = 361
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$s_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$s_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{361}=19$
$s_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-11)-19}{2*1}=\frac{-8}{2}
=-4
$
$s_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-11)+19}{2*1}=\frac{30}{2}
=15
$