20=x^2-6x

Solution for 20=x^2-6x equation:

20=x^2-6x

We move all terms to the left:

20-(x^2-6x)=0

We get rid of parentheses

-x^2+6x+20=0

We add all the numbers together, and all the variables

-1x^2+6x+20=0

a = -1; b = 6; c = +20;
Δ = b2-4ac
Δ = 62-4·(-1)·20
Δ = 116
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:

$\sqrt{\Delta}=\sqrt{116}=\sqrt{4*29}=\sqrt{4}*\sqrt{29}=2\sqrt{29}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(6)-2\sqrt{29}}{2*-1}=\frac{-6-2\sqrt{29}}{-2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(6)+2\sqrt{29}}{2*-1}=\frac{-6+2\sqrt{29}}{-2}
$