the answer:
perhaps the complement of question is as follow, otherwise, this is an example to show how to solve such a problem:
b is the midpoint of ac. if ab = 2x + 4 and BC = 3x + 6, find AC
B is the midpoint of ac implies AB=BC, and AC must be = AB+BC or BA+ CB in distance, so AC= 2x + 4 + 3x + 6= 5x+10