x(x+20)=800

Solution for x(x+20)=800 equation:

x(x+20)=800

We move all terms to the left:

x(x+20)-(800)=0

We multiply parentheses

x^2+20x-800=0

a = 1; b = 20; c = -800;
Δ = b2-4ac
Δ = 202-4·1·(-800)
Δ = 3600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3600}=60$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-60}{2*1}=\frac{-80}{2}
=-40
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+60}{2*1}=\frac{40}{2}
=20
$