All these figures are called polygons.
“Poly” means “many.”
What do you call a polygon of three sides? Of four sides? Of six sides? &c.
If the length of each side of triangle A is one inch, how long are the three sides together?
The sum of the sides of a polygon is its perimeter.
Which of the triangles has unequal sides? Which has equal sides?
The latter is called a regular polygon.
Which pentagon has one side longer than any one of its other sides?
Which has its sides all equal to each other? Are its angles also equal?
It is therefore a regular polygon, or regular pentagon.