402.12=3.14159*r^2

Solution for 402.12=3.14159*r^2 equation:

402.12=3.14159r^2

We move all terms to the left:

402.12-(3.14159r^2)=0

We get rid of parentheses

-3.14159r^2+402.12=0

a = -3.14159; b = 0; c = +402.12;
Δ = b2-4ac
Δ = 02-4·(-3.14159)·402.12
Δ = 5053.1846832
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-\sqrt{5053.1846832}}{2*-3.14159}=\frac{0-\sqrt{5053.1846832}}{-6.28318}
=-\frac{\sqrt{}}{-6.28318}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{5053.1846832}}{2*-3.14159}=\frac{0+\sqrt{5053.1846832}}{-6.28318}
=\frac{\sqrt{}}{-6.28318}
$