400=0.4x^2+36x-400

Solution for 400=0.4x^2+36x-400 equation:

400=0.4x^2+36x-400

We move all terms to the left:

400-(0.4x^2+36x-400)=0

We get rid of parentheses

-0.4x^2-36x+400+400=0

We add all the numbers together, and all the variables

-0.4x^2-36x+800=0

a = -0.4; b = -36; c = +800;
Δ = b2-4ac
Δ = -362-4·(-0.4)·800
Δ = 2576
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:

$\sqrt{\Delta}=\sqrt{2576}=\sqrt{16*161}=\sqrt{16}*\sqrt{161}=4\sqrt{161}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-36)-4\sqrt{161}}{2*-0.4}=\frac{36-4\sqrt{161}}{-0.8}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-36)+4\sqrt{161}}{2*-0.4}=\frac{36+4\sqrt{161}}{-0.8}
$