15=0.007x^2+0.003x

Solution for 15=0.007x^2+0.003x equation:

15=0.007x^2+0.003x

We move all terms to the left:

15-(0.007x^2+0.003x)=0

We get rid of parentheses

-0.007x^2-0.003x+15=0

a = -0.007; b = -0.003; c = +15;
Δ = b2-4ac
Δ = -0.0032-4·(-0.007)·15
Δ = 0.420009
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{-(-0.003)-\sqrt{0.420009}}{2*-0.007}=\frac{0.003-\sqrt{0.420009}}{-0.014}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-0.003)+\sqrt{0.420009}}{2*-0.007}=\frac{0.003+\sqrt{0.420009}}{-0.014}
$