400*(20-x)*(x-4)=0

Solution for 400*(20-x)*(x-4)=0 equation:

400(20-x)(x-4)=0

We add all the numbers together, and all the variables

400(-1x+20)(x-4)=0

We multiply parentheses ..

400(-1x^2+4x+20x-80)=0

We multiply parentheses

-400x^2+1600x+8000x-32000=0

We add all the numbers together, and all the variables

-400x^2+9600x-32000=0

a = -400; b = 9600; c = -32000;
Δ = b2-4ac
Δ = 96002-4·(-400)·(-32000)
Δ = 40960000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{40960000}=6400$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(9600)-6400}{2*-400}=\frac{-16000}{-800}
=+20
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(9600)+6400}{2*-400}=\frac{-3200}{-800}
=+4
$