Generally, we take relative velocity as

a-b

However, at higher velocities (specifically in the case of special relativity cases) it's

w = (a-b)/(1-ab/c^2)

or

w = (a+b)/(1+ab/c^2)

That's as far as I know. I had to take it for granted. Could someone take the time out and explain it to me? Thanks.
-Frog.