$\displaystyle \varphi'''-\lambda\varphi=0$

$\displaystyle m^3-\lambda=0\Rightarrow m^3=\lambda$

My book then does this:

$\displaystyle \displaystyle m=s, \ m=\left(\frac{-1\pm i\sqrt{3}}{2}\right)s,\ \ s=\lambda^{1/3}$

How were the roots of m determined?