90=-400x2+400x

Solution for 90=-400x2+400x equation:

90=-400x^2+400x

We move all terms to the left:

90-(-400x^2+400x)=0

We get rid of parentheses

400x^2-400x+90=0

a = 400; b = -400; c = +90;
Δ = b2-4ac
Δ = -4002-4·400·90
Δ = 16000
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:

$\sqrt{\Delta}=\sqrt{16000}=\sqrt{1600*10}=\sqrt{1600}*\sqrt{10}=40\sqrt{10}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-40\sqrt{10}}{2*400}=\frac{400-40\sqrt{10}}{800}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+40\sqrt{10}}{2*400}=\frac{400+40\sqrt{10}}{800}
$