216=f^2

Solution for 216=f^2 equation:

216=f^2

We move all terms to the left:

216-(f^2)=0

We add all the numbers together, and all the variables

-1f^2+216=0

a = -1; b = 0; c = +216;
Δ = b2-4ac
Δ = 02-4·(-1)·216
Δ = 864
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$f_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$f_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{864}=\sqrt{144*6}=\sqrt{144}*\sqrt{6}=12\sqrt{6}$
$f_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-12\sqrt{6}}{2*-1}=\frac{0-12\sqrt{6}}{-2}
=-\frac{12\sqrt{6}}{-2}
=-\frac{6\sqrt{6}}{-1}
$
$f_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+12\sqrt{6}}{2*-1}=\frac{0+12\sqrt{6}}{-2}
=\frac{12\sqrt{6}}{-2}
=\frac{6\sqrt{6}}{-1}
$