# What are the possible spin quantum numbers for an electron with m_l = -1?

Whichever you want: $- \frac{1}{2}$ or $+ \frac{1}{2}$. There was no ${m}_{s}$ given, which specifies the spin of the electron.
Note that by specifying ${m}_{l} = - 1$, you haven't explicitly shown what $l$ is. Since ${m}_{l} = \left\{- l , - l + 1 , . . . , 0 , . . . , l - 1 , l\right\}$, ${m}_{l} = - 1$ could correspond to any $l \ge 1$.
Either way, we do have some sort of orbital with ${m}_{l} = - 1$, whatever it may be. So, the electron has no particular spin other than the two possibilities of spin up or spin down.