So the teacher said it's like this OB - OA = AB and I don't understand it. Why aren't we doing this instead: OA - OB? aren't they the same thing?
Ask Tom and Jerry to go from O to B. Tell them that the start point is the tail and the end point is the head of the vector or arrow.
Tom goes directly to B and Jerry detours via point A before heading to B. Whether OB or OA+AB their effective effective displacement has remained the same.