(x-11)(x+15)=0

Solution for (x-11)(x+15)=0 equation:

(x-11)(x+15)=0

We multiply parentheses ..

(+x^2+15x-11x-165)=0

We get rid of parentheses

x^2+15x-11x-165=0

We add all the numbers together, and all the variables

x^2+4x-165=0

a = 1; b = 4; c = -165;
Δ = b2-4ac
Δ = 42-4·1·(-165)
Δ = 676
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{676}=26$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(4)-26}{2*1}=\frac{-30}{2}
=-15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(4)+26}{2*1}=\frac{22}{2}
=11
$