7=-0.004x^2+x

Solution for 7=-0.004x^2+x equation:

7=-0.004x^2+x

We move all terms to the left:

7-(-0.004x^2+x)=0

We get rid of parentheses

0.004x^2-x+7=0

We add all the numbers together, and all the variables

0.004x^2-1x+7=0

a = 0.004; b = -1; c = +7;
Δ = b2-4ac
Δ = -12-4·0.004·7
Δ = 0.888
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{-(-1)-\sqrt{0.888}}{2*0.004}=\frac{1-\sqrt{0.888}}{0.008}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+\sqrt{0.888}}{2*0.004}=\frac{1+\sqrt{0.888}}{0.008}
$