81*3.14=3.14*r^2

Solution for 81*3.14=3.14*r^2 equation:

81*3.14=3.14r^2

We move all terms to the left:

81*3.14-(3.14r^2)=0

We add all the numbers together, and all the variables

-(3.14r^2)+254.34=0

We get rid of parentheses

-3.14r^2+254.34=0

a = -3.14; b = 0; c = +254.34;
Δ = b2-4ac
Δ = 02-4·(-3.14)·254.34
Δ = 3194.5104
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{3194.5104}}{2*-3.14}=\frac{0-\sqrt{3194.5104}}{-6.28}
=-\frac{\sqrt{}}{-6.28}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{3194.5104}}{2*-3.14}=\frac{0+\sqrt{3194.5104}}{-6.28}
=\frac{\sqrt{}}{-6.28}
$