I=-20x2+400x

Solution for I=-20x2+400x equation:

=-20I^2+400I

We move all terms to the left:

-(-20I^2+400I)=0

We get rid of parentheses

20I^2-400I=0

a = 20; b = -400; c = 0;
Δ = b2-4ac
Δ = -4002-4·20·0
Δ = 160000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$I_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$I_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{160000}=400$
$I_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-400}{2*20}=\frac{0}{40}
=0
$
$I_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+400}{2*20}=\frac{800}{40}
=20
$