Y=400000-1000x2

Solution for Y=400000-1000x2 equation:

=400000-1000Y^2

We move all terms to the left:

-(400000-1000Y^2)=0

We get rid of parentheses

1000Y^2-400000=0

a = 1000; b = 0; c = -400000;
Δ = b2-4ac
Δ = 02-4·1000·(-400000)
Δ = 1600000000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1600000000}=40000$
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-40000}{2*1000}=\frac{-40000}{2000}
=-20
$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+40000}{2*1000}=\frac{40000}{2000}
=20
$