In the given figure Lines NM & PO are parallel .From statement 2 and 3 we have <2=<3 and <1=<3.
The Transitive property states: If two sides or angles are equal to one another and one of them is equal to third side or angle then the first side or angle is equal to the third angle or side .The formula for this property is if a = b and b = c, then a = c.
So if <2=<3 and <1=<3 then by Transitive property <1=<2.
The missing reason in the proof is Transitive property.