Answer:
The error was in step 3
Step-by-step explanation:
The first two steps are correct, the property of addition of internal angles of a triangle that equal [tex]180^o[/tex] is properly used, Also the addition of supplemental angles to total [tex]180^o[/tex] is also properly used.
In step 3. there is an error. Notice how it should have been done:
[tex]\angle m+\angle n +\angle o=180^o\\\angle o + \angle p = 180^o\\then:\\\angle m+\angle n +\angle o=180^o=\angle o+\angle p\\\angle m+\angle n +\angle o=\angle o+\angle p[/tex]
