If the points A, B, C are on the same line, and B lies in between A and C, then AC is the sum of AB and BC. So we'd get AC = 15. On the other hand, if A lies in between B and C, then AC is the difference BC - AB, which gives us AC = 5.
If the points A, B, C are not collinear, then the AC length will depend on the angle that's between AB and BC. Read more