Answer:
v a/c = v a/b + v b/c
Step-by-step explanation:
Assuming you are saying Velocity of A relative to C as v a/c (I'm sorry I have not seen this format in over local texts )
Using the triangle of velocity you can arrive at this equation,
Velocity of A relative to C = Velocity of A relative to B + Velocity of B relative to C.
Consider a car travelling on Earth with a passenger while you walk on the road. So your velocity as seen by passenger is,
Velocity of you relative to passenger = Velocity of you relative to Earth + Velocity of Earth relative to passenger.