So the definition of a regular polygon is that the sides are all equal, the interior angles are all equal, and the exterior angles are all equal.
Notice how with #s 3 and 4, the angles are all shown as equal, as well as their sides. However, #s 5 and 6 have different angles and sides, respectively.
Therefore 3 and 4 are regular, but 5 and 6 are irregular (not regular).
