here, PN is the bisector of line MO and it divides the line MO into the two equal parts,
Since PN bisects MO that means that MN= NO because of the definition of the bisect, because PN perpendicular MO that means that angle ONP and angle MNP are both right angles because of the definition of perpendicular, this means that angle ONP = angle MNP because all right angles are congruent .also by the reflexive property of the congruence line PN = line PN , based on these three pairs of congruent corresponding parts triangle ONP = triangle MNP based on the SAS triangle congruence postulate.