400=r2

Solution for 400=r2 equation:

400=r2

We move all terms to the left:

400-(r2)=0

We add all the numbers together, and all the variables

-1r^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:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

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