And we do not know the answer to that.
In general here is a outline of a proof.
Suppose f is a coordinate system on line AB.
So f(A) is the coordinate of point A.
If f(B)-f(A)>0 then define g(x)=f(x)-f(A) otherwise define g(x)=f(A)-f(x).
By use of the betweenness theorems the result should follow.