225=3.14r^2

Solution for 225=3.14r^2 equation:

225=3.14r^2

We move all terms to the left:

225-(3.14r^2)=0

We get rid of parentheses

-3.14r^2+225=0

a = -3.14; b = 0; c = +225;
Δ = b2-4ac
Δ = 02-4·(-3.14)·225
Δ = 2826
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{2826}=\sqrt{9*314}=\sqrt{9}*\sqrt{314}=3\sqrt{314}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-3\sqrt{314}}{2*-3.14}=\frac{0-3\sqrt{314}}{-6.28}
=-\frac{3\sqrt{314}}{-6.28}
=-\frac{\sqrt{314}}{-2.0933333333333}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+3\sqrt{314}}{2*-3.14}=\frac{0+3\sqrt{314}}{-6.28}
=\frac{3\sqrt{314}}{-6.28}
=\frac{\sqrt{314}}{-2.0933333333333}
$