What is the relative velocity equation?
If an object A moves with velocity
As an example, let us consider linear motion for simplicity and assume that our observations in one dimension holds for two and three dimensions. (By using vector notation, this happily turns out to be the case.)
Two cars A and B moving with velocities
Velocity of A as observed by a person sitting in car B is then naturally,
The observer sees the car A going away (ahead) from it with speed
If the opposite is the case,
The car B goes ahead of A with the speed
Extending what we observed here to three dimensions is trivial.
We just have to use vector notations for that. Other details remain unchanged.