For n=4, l=2, ml=-2, we could suggest that the orbital is 4d.
That's because n=4 represents the energy level which is 4 and l=2 takes "d" as an orbital.
Now, if ml=-2, we got that:
If ml=-2, only this set is possible,
The magnetic quantum number tells us how many orbitals there are in a subshell and the possible orientations of these orbitals.
For example, let's suppose that you want to find the magnetic quantum number for C.
What we do first, is to state the electron configuration:
The last term is going to tell us the quantum numbers.
n=2 since it ends with coefficient 2.
l=1 because it ends with p.
Now, to find the value of ml, what we do is to draw three boxes and put the electrons with arrows: (We're going to call the middle box as zero and we're going to start counting the boxes next to it with positive numbers to the right and negative numbers to the left).
Then, we're going to organize the two electrons in the boxes starting to put them up:
We finished to organize them in position zero, so, ml=0. That's because there's one set, since the orientation could be just that one.