0=-0.008x^2+0.4x+40

Solution for 0=-0.008x^2+0.4x+40 equation:

0=-0.008x^2+0.4x+40

We move all terms to the left:

0-(-0.008x^2+0.4x+40)=0

We add all the numbers together, and all the variables

-(-0.008x^2+0.4x+40)=0

We get rid of parentheses

0.008x^2-0.4x-40=0

a = 0.008; b = -0.4; c = -40;
Δ = b2-4ac
Δ = -0.42-4·0.008·(-40)
Δ = 1.44
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{-(-0.4)-\sqrt{1.44}}{2*0.008}=\frac{0.4-\sqrt{1.44}}{0.016}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-0.4)+\sqrt{1.44}}{2*0.008}=\frac{0.4+\sqrt{1.44}}{0.016}
$