20=-8k+k^2

Solution for 20=-8k+k^2 equation:

20=-8k+k^2

We move all terms to the left:

20-(-8k+k^2)=0

We get rid of parentheses

-k^2+8k+20=0

We add all the numbers together, and all the variables

-1k^2+8k+20=0

a = -1; b = 8; c = +20;
Δ = b2-4ac
Δ = 82-4·(-1)·20
Δ = 144
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$k_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$k_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{144}=12$
$k_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(8)-12}{2*-1}=\frac{-20}{-2}
=+10
$
$k_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(8)+12}{2*-1}=\frac{4}{-2}
=-2
$