99=x^2+2x

Solution for 99=x^2+2x equation:

99=x^2+2x

We move all terms to the left:

99-(x^2+2x)=0

We get rid of parentheses

-x^2-2x+99=0

We add all the numbers together, and all the variables

-1x^2-2x+99=0

a = -1; b = -2; c = +99;
Δ = b2-4ac
Δ = -22-4·(-1)·99
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-2)-20}{2*-1}=\frac{-18}{-2}
=+9
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-2)+20}{2*-1}=\frac{22}{-2}
=-11
$