.20=0.0021x^2-0.286x=16.28

Solution for .20=0.0021x^2-0.286x=16.28 equation:

.20=0.0021x^2-0.286x=16.28

We move all terms to the left:

.20-(0.0021x^2-0.286x)=0

We add all the numbers together, and all the variables

-(0.0021x^2-0.286x)+0.2=0

We get rid of parentheses

-0.0021x^2+0.286x+0.2=0

a = -0.0021; b = 0.286; c = +0.2;
Δ = b2-4ac
Δ = 0.2862-4·(-0.0021)·0.2
Δ = 0.083476
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{-(0.286)-\sqrt{0.083476}}{2*-0.0021}=\frac{-0.286-\sqrt{0.083476}}{-0.0042}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.286)+\sqrt{0.083476}}{2*-0.0021}=\frac{-0.286+\sqrt{0.083476}}{-0.0042}
$