(5)=x^2-21x

Solution for (5)=x^2-21x equation:

(5)=x^2-21x

We move all terms to the left:

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

We get rid of parentheses

-x^2+21x+5=0

We add all the numbers together, and all the variables

-1x^2+21x+5=0

a = -1; b = 21; c = +5;
Δ = b2-4ac
Δ = 212-4·(-1)·5
Δ = 461
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-\sqrt{461}}{2*-1}=\frac{-21-\sqrt{461}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+\sqrt{461}}{2*-1}=\frac{-21+\sqrt{461}}{-2}
$