Answer:
1. Given
2. Segment Addition Postulate
3. Substitution/Transitive...
4. Subtraction Property...
5. Symmetric Property...
Explanation:
1. Notice that the statement is identical to the "Given:"
2. Look at the figure. B is between A and C, so segment AB + segment BC = segment AC.
3. Look at statements 1 and 2. Since AC=AB+AB and AC=AB+BC, you can set AB+AB=AB+BC.
4. Look at the equation in step 3. Subtract AB from both sides.
5. Notice that BC=AB from statement 4 is just switched.
In general, every next statement in a proof will refer to something that's already stated - whether in the Given or the previous 1-3 statements.