20.5b=0.5b^2

Solution for 20.5b=0.5b^2 equation:

20.5b=0.5b^2

We move all terms to the left:

20.5b-(0.5b^2)=0

We get rid of parentheses

-0.5b^2+20.5b=0

a = -0.5; b = 20.5; c = 0;
Δ = b2-4ac
Δ = 20.52-4·(-0.5)·0
Δ = 420.25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(20.5)-\sqrt{420.25}}{2*-0.5}=\frac{-20.5-\sqrt{420.25}}{-1}
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(20.5)+\sqrt{420.25}}{2*-0.5}=\frac{-20.5+\sqrt{420.25}}{-1}
$