8=0.4x^2-2x

Solution for 8=0.4x^2-2x equation:

8=0.4x^2-2x

We move all terms to the left:

8-(0.4x^2-2x)=0

We get rid of parentheses

-0.4x^2+2x+8=0

a = -0.4; b = 2; c = +8;
Δ = b2-4ac
Δ = 22-4·(-0.4)·8
Δ = 16.8
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{-(2)-\sqrt{16.8}}{2*-0.4}=\frac{-2-\sqrt{16.8}}{-0.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(2)+\sqrt{16.8}}{2*-0.4}=\frac{-2+\sqrt{16.8}}{-0.8}
$