7000=-0.001x^2+8x

Solution for 7000=-0.001x^2+8x equation:

7000=-0.001x^2+8x

We move all terms to the left:

7000-(-0.001x^2+8x)=0

We get rid of parentheses

0.001x^2-8x+7000=0

a = 0.001; b = -8; c = +7000;
Δ = b2-4ac
Δ = -82-4·0.001·7000
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-6}{2*0.001}=\frac{2}{0.002}
=1000
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+6}{2*0.001}=\frac{14}{0.002}
=7000
$