Question

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

Answer 1

*assuming that ABC is a straight line** 
then you have 
AB = 17.89 ..... this is the length of the whole line 

BC is a part of the line = 7 

therefore the length of the other part = 17.89 - 7 

= 10.89

Answer 2

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