(N/A) The terms that need to be defined are:
$1$. Polygon: $A$ simple closed figure made up of three or more line segments.
$2$. Line segment: $A$ part of a line with two end points.
$3$. Angle: $A$ figure formed by two rays with a common initial point.
Undefined terms used in the definition are: Line,Point,Part.
Justification:
Given that two line segments are equal to the third one,by the property of transitivity,all three sides of the equilateral triangle are equal.
Since all angles are $60^{\circ}$ each,they are equal to one another. According to Euclid's first axiom,things which are equal to the same thing are equal to one another. Thus,all angles are equal.