(5x-4)(-4x-1)=0

Solution for (5x-4)(-4x-1)=0 equation:

(5x-4)(-4x-1)=0

We multiply parentheses ..

(-20x^2-5x+16x+4)=0

We get rid of parentheses

-20x^2-5x+16x+4=0

We add all the numbers together, and all the variables

-20x^2+11x+4=0

a = -20; b = 11; c = +4;
Δ = b2-4ac
Δ = 112-4·(-20)·4
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(11)-21}{2*-20}=\frac{-32}{-40}
=4/5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(11)+21}{2*-20}=\frac{10}{-40}
=-1/4
$