x2-2x-255=0.

Solution for x2-2x-255=0. equation:

x2-2x-255=0.

We add all the numbers together, and all the variables

x^2-2x-255=0

a = 1; b = -2; c = -255;
Δ = b2-4ac
Δ = -22-4·1·(-255)
Δ = 1024
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1024}=32$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-32}{2*1}=\frac{-30}{2}
=-15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+32}{2*1}=\frac{34}{2}
=17
$