900=0.15x^2

Solution for 900=0.15x^2 equation:

900=0.15x^2

We move all terms to the left:

900-(0.15x^2)=0

We get rid of parentheses

-0.15x^2+900=0

a = -0.15; b = 0; c = +900;
Δ = b2-4ac
Δ = 02-4·(-0.15)·900
Δ = 540
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:

$\sqrt{\Delta}=\sqrt{540}=\sqrt{36*15}=\sqrt{36}*\sqrt{15}=6\sqrt{15}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-6\sqrt{15}}{2*-0.15}=\frac{0-6\sqrt{15}}{-0.3}
=-\frac{6\sqrt{15}}{-0.3}
=-\frac{\sqrt{15}}{-0.05}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+6\sqrt{15}}{2*-0.15}=\frac{0+6\sqrt{15}}{-0.3}
=\frac{6\sqrt{15}}{-0.3}
=\frac{\sqrt{15}}{-0.05}
$