0.64=r2

Solution for 0.64=r2 equation:

0.64=r2

We move all terms to the left:

0.64-(r2)=0

We add all the numbers together, and all the variables

-1r^2+0.64=0

a = -1; b = 0; c = +0.64;
Δ = b2-4ac
Δ = 02-4·(-1)·0.64
Δ = 2.56
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{2.56}}{2*-1}=\frac{0-\sqrt{2.56}}{-2}
=-\frac{\sqrt{}}{-2}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{2.56}}{2*-1}=\frac{0+\sqrt{2.56}}{-2}
=\frac{\sqrt{}}{-2}
$