x(x-400)=0

Solution for x(x-400)=0 equation:

x(x-400)=0

We multiply parentheses

x^2-400x=0

a = 1; b = -400; c = 0;
Δ = b2-4ac
Δ = -4002-4·1·0
Δ = 160000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{160000}=400$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-400}{2*1}=\frac{0}{2}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+400}{2*1}=\frac{800}{2}
=400
$