(1600-5x)(80+x)=0

Solution for (1600-5x)(80+x)=0 equation:

(1600-5x)(80+x)=0

We add all the numbers together, and all the variables

(-5x+1600)(x+80)=0

We multiply parentheses ..

(-5x^2-400x+1600x+128000)=0

We get rid of parentheses

-5x^2-400x+1600x+128000=0

We add all the numbers together, and all the variables

-5x^2+1200x+128000=0

a = -5; b = 1200; c = +128000;
Δ = b2-4ac
Δ = 12002-4·(-5)·128000
Δ = 4000000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{4000000}=2000$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1200)-2000}{2*-5}=\frac{-3200}{-10}
=+320
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1200)+2000}{2*-5}=\frac{800}{-10}
=-80
$