Calculate protein concentration (x) from known absorbance

Question

Got known absorbance ($y$) and I want to find $x$ from this formula:

$$y = -4\times 10^{-7} x^2 + 0.001 x + 0.2529$$

Answer 1

Went for the most obvious answer and solved this using quadratic formula as it's faster than finding x^2...

$$ x_{1,2} = \frac{-b\pm\sqrt{b^2-4ac}}{2a} $$