1000=80*x-0.4*x^2

Solution for 1000=80*x-0.4*x^2 equation:

1000=80x-0.4x^2

We move all terms to the left:

1000-(80x-0.4x^2)=0

We get rid of parentheses

0.4x^2-80x+1000=0

a = 0.4; b = -80; c = +1000;
Δ = b2-4ac
Δ = -802-4·0.4·1000
Δ = 4800
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{4800}=\sqrt{1600*3}=\sqrt{1600}*\sqrt{3}=40\sqrt{3}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-80)-40\sqrt{3}}{2*0.4}=\frac{80-40\sqrt{3}}{0.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-80)+40\sqrt{3}}{2*0.4}=\frac{80+40\sqrt{3}}{0.8}
$