Quantum numbers are n, l, m(sub.l), m(sub.s). Confusing, but heres the gist of it:
n represents your row number. Chlorine is in the 3rd row, so n=3. Easy.
"L" can be a range of numbers from 0 to one less than n (n=3, so L can range from 0 to 2). These numbers (0,1,2) represent the possible shells a valence chlorine electron can reside. 0=s 1=p, 2=d. Since we're dealing with the 13th electron, we're in a P orbital (so n=1)
"MsubL" represents the possible sub-shells the valence chlorine electron can reside. Numerically, MsubL=+ or -(L). Since we're looking at a P orbital, L=1. Thus, each sub-shell correlates to -1, 0, and 1. Then we fill the subshells with 1 electron each before doubling up (sounds familiar?). Since the 13th electron is the only electron in our P orbital, its gonna end up in the -1 sub-shell. So... MsubL=-1.
MsubS is easy. Its just the electron spin (what are thooseee). Who knows, and who cares. Just remember its either +1/2 or -1/2.
So, to summarize:
N = 3
L = 1
MsubL = -1
MsubS = +/- 1/2
Pretty sure you knew all this, but I'm just covering bases here. And heres my point:
Note how L and MsubL would have been completely different if we were asked to solve for the 11th or 12th electron.
Good luck.