4=3.141592*r^2

Solution for 4=3.141592*r^2 equation:

4=3.141592*r^2

We move all terms to the left:

4-(3.141592*r^2)=0

We get rid of parentheses

-3.141592*r^2+4=0

We add all the numbers together, and all the variables

-3.141592r^2+4=0

a = -3.141592; b = 0; c = +4;
Δ = b2-4ac
Δ = 02-4·(-3.141592)·4
Δ = 50.265472
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{50.265472}}{2*-3.141592}=\frac{0-\sqrt{50.265472}}{-6.283184}
=-\frac{\sqrt{}}{-6.283184}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{50.265472}}{2*-3.141592}=\frac{0+\sqrt{50.265472}}{-6.283184}
=\frac{\sqrt{}}{-6.283184}
$