We have to prove that if the side of one equilateral triangle is equal to the side of another equilateral triangle, then the triangles are congruent.
That is AB=DE (GIVEN )
Angles opposite to equal sides of a triangle are equal.
Angle A =Angle D
Each angle of an equilateral triangle is of 60°.
If three sides of one triangle are equal to three sides of the other triangle, then the two triangles are congruent (SSS Congruence Rule).