(c+15)(c-3)=0

Solution for (c+15)(c-3)=0 equation:

(c+15)(c-3)=0

We multiply parentheses ..

(+c^2-3c+15c-45)=0

We get rid of parentheses

c^2-3c+15c-45=0

We add all the numbers together, and all the variables

c^2+12c-45=0

a = 1; b = 12; c = -45;
Δ = b2-4ac
Δ = 122-4·1·(-45)
Δ = 324
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{324}=18$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(12)-18}{2*1}=\frac{-30}{2}
=-15
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(12)+18}{2*1}=\frac{6}{2}
=3
$