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 = 304000
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:

$\sqrt{\Delta}=\sqrt{304000}=\sqrt{1600*190}=\sqrt{1600}*\sqrt{190}=40\sqrt{190}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-40\sqrt{190}}{2*-400}=\frac{400-40\sqrt{190}}{-800}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+40\sqrt{190}}{2*-400}=\frac{400+40\sqrt{190}}{-800}
$