(x-11)(x+9)=0

Solution for (x-11)(x+9)=0 equation:

(x-11)(x+9)=0

We multiply parentheses ..

(+x^2+9x-11x-99)=0

We get rid of parentheses

x^2+9x-11x-99=0

We add all the numbers together, and all the variables

x^2-2x-99=0

a = 1; b = -2; c = -99;
Δ = b2-4ac
Δ = -22-4·1·(-99)
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-20}{2*1}=\frac{-18}{2}
=-9
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+20}{2*1}=\frac{22}{2}
=11
$