900=x^2+80x

Solution for 900=x^2+80x equation:

900=x^2+80x

We move all terms to the left:

900-(x^2+80x)=0

We get rid of parentheses

-x^2-80x+900=0

We add all the numbers together, and all the variables

-1x^2-80x+900=0

a = -1; b = -80; c = +900;
Δ = b2-4ac
Δ = -802-4·(-1)·900
Δ = 10000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{10000}=100$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-80)-100}{2*-1}=\frac{-20}{-2}
=+10
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-80)+100}{2*-1}=\frac{180}{-2}
=-90
$