-5x(4+5x)=0

Solution for -5x(4+5x)=0 equation:

-5x(4+5x)=0

We add all the numbers together, and all the variables

-5x(5x+4)=0

We multiply parentheses

-25x^2-20x=0

a = -25; b = -20; c = 0;
Δ = b2-4ac
Δ = -202-4·(-25)·0
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-20}{2*-25}=\frac{0}{-50}
=0
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20}{2*-25}=\frac{40}{-50}
=-4/5
$