21=c2+15

Solution for 21=c2+15 equation:

21=c2+15

We move all terms to the left:

21-(c2+15)=0

We add all the numbers together, and all the variables

-(+c^2+15)+21=0

We get rid of parentheses

-c^2-15+21=0

We add all the numbers together, and all the variables

-1c^2+6=0

a = -1; b = 0; c = +6;
Δ = b2-4ac
Δ = 02-4·(-1)·6
Δ = 24
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{24}=\sqrt{4*6}=\sqrt{4}*\sqrt{6}=2\sqrt{6}$
$c_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{6}}{2*-1}=\frac{0-2\sqrt{6}}{-2}
=-\frac{2\sqrt{6}}{-2}
=-\frac{\sqrt{6}}{-1}
$
$c_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{6}}{2*-1}=\frac{0+2\sqrt{6}}{-2}
=\frac{2\sqrt{6}}{-2}
=\frac{\sqrt{6}}{-1}
$