21=d^2

Solution for 21=d^2 equation:

21=d^2

We move all terms to the left:

21-(d^2)=0

We add all the numbers together, and all the variables

-1d^2+21=0

a = -1; b = 0; c = +21;
Δ = b2-4ac
Δ = 02-4·(-1)·21
Δ = 84
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{84}=\sqrt{4*21}=\sqrt{4}*\sqrt{21}=2\sqrt{21}$
$d_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{21}}{2*-1}=\frac{0-2\sqrt{21}}{-2}
=-\frac{2\sqrt{21}}{-2}
=-\frac{\sqrt{21}}{-1}
$
$d_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{21}}{2*-1}=\frac{0+2\sqrt{21}}{-2}
=\frac{2\sqrt{21}}{-2}
=\frac{\sqrt{21}}{-1}
$