-20=2000+100x-0.1x^2

Solution for -20=2000+100x-0.1x^2 equation:

-20=2000+100x-0.1x^2

We move all terms to the left:

-20-(2000+100x-0.1x^2)=0

We get rid of parentheses

0.1x^2-100x-2000-20=0

We add all the numbers together, and all the variables

0.1x^2-100x-2020=0

a = 0.1; b = -100; c = -2020;
Δ = b2-4ac
Δ = -1002-4·0.1·(-2020)
Δ = 10808
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{10808}=\sqrt{4*2702}=\sqrt{4}*\sqrt{2702}=2\sqrt{2702}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-100)-2\sqrt{2702}}{2*0.1}=\frac{100-2\sqrt{2702}}{0.2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-100)+2\sqrt{2702}}{2*0.1}=\frac{100+2\sqrt{2702}}{0.2}
$