Definition of Linear Pairs of Angles; and Definition of supplementary angles.
Step-by-step explanation:
A linear pair is two angles that form a straight line. Thus our second step, that ∠MNP and ∠PNO are a linear pair, is from the definition of linear pairs.
We know from our third step that linear pairs are supplementary. By the definition of supplementary angles, this means they have a sum of 180°.