Answer: Substitution, Transitive
If <1 = <2 and <2 = <3, there are two properties that let you prove <1 = <3.
The transitive property states that if a = b and b = c, then a = c by the transitive property.
You can also substitute <3 for <2 in <1 = <2 as they are congruent, therefore substitution is also another way to do it.