5x(x-4)=40

Solution for 5x(x-4)=40 equation:

5x(x-4)=40

We move all terms to the left:

5x(x-4)-(40)=0

We multiply parentheses

5x^2-20x-40=0

a = 5; b = -20; c = -40;
Δ = b2-4ac
Δ = -202-4·5·(-40)
Δ = 1200
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:

$\sqrt{\Delta}=\sqrt{1200}=\sqrt{400*3}=\sqrt{400}*\sqrt{3}=20\sqrt{3}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-20\sqrt{3}}{2*5}=\frac{20-20\sqrt{3}}{10}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20\sqrt{3}}{2*5}=\frac{20+20\sqrt{3}}{10}
$