The question doesn't say that the points A, B, C and D are in that order along the line. They might be in the order A, C, D, B for instance.
So if AC = 6 units and CD = 3, then there are two possible positions for D - one on either side of C. I've called these and .
In the same way, for each of the possible positions of D, there are 2 possible positions for B - 4 positions altogether. I've called these through .
Do you see why there are several possible answers now?