4x-3x(20-x)=4

Solution for 4x-3x(20-x)=4 equation:

4x-3x(20-x)=4

We move all terms to the left:

4x-3x(20-x)-(4)=0

We add all the numbers together, and all the variables

4x-3x(-1x+20)-4=0

We multiply parentheses

3x^2+4x-60x-4=0

We add all the numbers together, and all the variables

3x^2-56x-4=0

a = 3; b = -56; c = -4;
Δ = b2-4ac
Δ = -562-4·3·(-4)
Δ = 3184
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{3184}=\sqrt{16*199}=\sqrt{16}*\sqrt{199}=4\sqrt{199}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-56)-4\sqrt{199}}{2*3}=\frac{56-4\sqrt{199}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-56)+4\sqrt{199}}{2*3}=\frac{56+4\sqrt{199}}{6}
$