400=0.10x^2

Solution for 400=0.10x^2 equation:

400=0.10x^2

We move all terms to the left:

400-(0.10x^2)=0

We get rid of parentheses

-0.10x^2+400=0

We add all the numbers together, and all the variables

-0.1x^2+400=0

a = -0.1; b = 0; c = +400;
Δ = b2-4ac
Δ = 02-4·(-0.1)·400
Δ = 160
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:

$\sqrt{\Delta}=\sqrt{160}=\sqrt{16*10}=\sqrt{16}*\sqrt{10}=4\sqrt{10}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-4\sqrt{10}}{2*-0.1}=\frac{0-4\sqrt{10}}{-0.2}
=-\frac{4\sqrt{10}}{-0.2}
=-\frac{\sqrt{10}}{-0.05}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+4\sqrt{10}}{2*-0.1}=\frac{0+4\sqrt{10}}{-0.2}
=\frac{4\sqrt{10}}{-0.2}
=\frac{\sqrt{10}}{-0.05}
$