x^2-2x=399

Solution for x^2-2x=399 equation:

x^2-2x=399

We move all terms to the left:

x^2-2x-(399)=0

a = 1; b = -2; c = -399;
Δ = b2-4ac
Δ = -22-4·1·(-399)
Δ = 1600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600}=40$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-40}{2*1}=\frac{-38}{2}
=-19
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+40}{2*1}=\frac{42}{2}
=21
$