81=3.14r^2

Solution for 81=3.14r^2 equation:

81=3.14r^2

We move all terms to the left:

81-(3.14r^2)=0

We get rid of parentheses

-3.14r^2+81=0

a = -3.14; b = 0; c = +81;
Δ = b2-4ac
Δ = 02-4·(-3.14)·81
Δ = 1017.36
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{1017.36}}{2*-3.14}=\frac{0-\sqrt{1017.36}}{-6.28}
=-\frac{\sqrt{}}{-6.28}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{1017.36}}{2*-3.14}=\frac{0+\sqrt{1017.36}}{-6.28}
=\frac{\sqrt{}}{-6.28}
$