225=r^2

Solution for 225=r^2 equation:

225=r^2

We move all terms to the left:

225-(r^2)=0

We add all the numbers together, and all the variables

-1r^2+225=0

a = -1; b = 0; c = +225;
Δ = b2-4ac
Δ = 02-4·(-1)·225
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-30}{2*-1}=\frac{-30}{-2}
=+15
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+30}{2*-1}=\frac{30}{-2}
=-15
$