5=3.15r2

Solution for 5=3.15r2 equation:

5=3.15r^2

We move all terms to the left:

5-(3.15r^2)=0

We get rid of parentheses

-3.15r^2+5=0

a = -3.15; b = 0; c = +5;
Δ = b2-4ac
Δ = 02-4·(-3.15)·5
Δ = 63
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{63}=\sqrt{9*7}=\sqrt{9}*\sqrt{7}=3\sqrt{7}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-3\sqrt{7}}{2*-3.15}=\frac{0-3\sqrt{7}}{-6.3}
=-\frac{3\sqrt{7}}{-6.3}
=-\frac{\sqrt{7}}{-2.1}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+3\sqrt{7}}{2*-3.15}=\frac{0+3\sqrt{7}}{-6.3}
=\frac{3\sqrt{7}}{-6.3}
=\frac{\sqrt{7}}{-2.1}
$