Given that BC = 7, AB = 17.89, and C is between A and B, what is the length of?

17.89-7=10.89

You know that the distance from A to b is 17.89 and the distance from b to c is 7. So the length from a to c is the whole distance (a to b) subtracted by a portion of the distance (b to c) giving us 10.89