0=x^2-21-81

Solution for 0=x^2-21-81 equation:

0=x^2-21-81

We move all terms to the left:

0-(x^2-21-81)=0

We add all the numbers together, and all the variables

-(x^2-21-81)=0

We get rid of parentheses

-x^2+21+81=0

We add all the numbers together, and all the variables

-1x^2+102=0

a = -1; b = 0; c = +102;
Δ = b2-4ac
Δ = 02-4·(-1)·102
Δ = 408
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{408}=\sqrt{4*102}=\sqrt{4}*\sqrt{102}=2\sqrt{102}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{102}}{2*-1}=\frac{0-2\sqrt{102}}{-2}
=-\frac{2\sqrt{102}}{-2}
=-\frac{\sqrt{102}}{-1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{102}}{2*-1}=\frac{0+2\sqrt{102}}{-2}
=\frac{2\sqrt{102}}{-2}
=\frac{\sqrt{102}}{-1}
$