(x)(x+2)=399

Solution for (x)(x+2)=399 equation:

(x)(x+2)=399

We move all terms to the left:

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

We multiply parentheses

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