x(x-4)=396

Solution for x(x-4)=396 equation:

x(x-4)=396

We move all terms to the left:

x(x-4)-(396)=0

We multiply parentheses

x^2-4x-396=0

a = 1; b = -4; c = -396;
Δ = b2-4ac
Δ = -42-4·1·(-396)
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{-(-4)-40}{2*1}=\frac{-36}{2}
=-18
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4)+40}{2*1}=\frac{44}{2}
=22
$