15=3.14*r^2

Solution for 15=3.14*r^2 equation:

15=3.14r^2

We move all terms to the left:

15-(3.14r^2)=0

We get rid of parentheses

-3.14r^2+15=0

a = -3.14; b = 0; c = +15;
Δ = b2-4ac
Δ = 02-4·(-3.14)·15
Δ = 188.4
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{188.4}}{2*-3.14}=\frac{0-\sqrt{188.4}}{-6.28}
=-\frac{\sqrt{}}{-6.28}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{188.4}}{2*-3.14}=\frac{0+\sqrt{188.4}}{-6.28}
=\frac{\sqrt{}}{-6.28}
$