20=0.25x2-8x

Solution for 20=0.25x2-8x equation:

20=0.25x^2-8x

We move all terms to the left:

20-(0.25x^2-8x)=0

We get rid of parentheses

-0.25x^2+8x+20=0

a = -0.25; b = 8; c = +20;
Δ = b2-4ac
Δ = 82-4·(-0.25)·20
Δ = 84
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:

$\sqrt{\Delta}=\sqrt{84}=\sqrt{4*21}=\sqrt{4}*\sqrt{21}=2\sqrt{21}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(8)-2\sqrt{21}}{2*-0.25}=\frac{-8-2\sqrt{21}}{-0.5}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(8)+2\sqrt{21}}{2*-0.25}=\frac{-8+2\sqrt{21}}{-0.5}
$