0=d^2-20d-80

Solution for 0=d^2-20d-80 equation:

0=d^2-20d-80

We move all terms to the left:

0-(d^2-20d-80)=0

We add all the numbers together, and all the variables

-(d^2-20d-80)=0

We get rid of parentheses

-d^2+20d+80=0

We add all the numbers together, and all the variables

-1d^2+20d+80=0

a = -1; b = 20; c = +80;
Δ = b2-4ac
Δ = 202-4·(-1)·80
Δ = 720
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{720}=\sqrt{144*5}=\sqrt{144}*\sqrt{5}=12\sqrt{5}$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20)-12\sqrt{5}}{2*-1}=\frac{-20-12\sqrt{5}}{-2}
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20)+12\sqrt{5}}{2*-1}=\frac{-20+12\sqrt{5}}{-2}
$