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

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

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

We add all the numbers together, and all the variables

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

We multiply parentheses ..

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

We get rid of parentheses

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

We add all the numbers together, and all the variables

-5x^2-24x-16=0

a = -5; b = -24; c = -16;
Δ = b2-4ac
Δ = -242-4·(-5)·(-16)
Δ = 256
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{256}=16$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-24)-16}{2*-5}=\frac{8}{-10}
=-4/5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-24)+16}{2*-5}=\frac{40}{-10}
=-4
$