400=x2

Solution for 400=x2 equation:

400=x2

We move all terms to the left:

400-(x2)=0

We add all the numbers together, and all the variables

-1x^2+400=0

a = -1; b = 0; c = +400;
Δ = b2-4ac
Δ = 02-4·(-1)·400
Δ = 1600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600}=40$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-40}{2*-1}=\frac{-40}{-2}
=+20
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+40}{2*-1}=\frac{40}{-2}
=-20
$