30=0.1x+0.001x^2

Solution for 30=0.1x+0.001x^2 equation:

30=0.1x+0.001x^2

We move all terms to the left:

30-(0.1x+0.001x^2)=0

We get rid of parentheses

-0.001x^2-0.1x+30=0

a = -0.001; b = -0.1; c = +30;
Δ = b2-4ac
Δ = -0.12-4·(-0.001)·30
Δ = 0.13
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-0.1)-\sqrt{0.13}}{2*-0.001}=\frac{0.1-\sqrt{0.13}}{-0.002}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-0.1)+\sqrt{0.13}}{2*-0.001}=\frac{0.1+\sqrt{0.13}}{-0.002}
$