What is the base of the expression

Which triangle congruence postulate or theorem justifies my claim that the steps i made using construction paper and scissors produce an isosceles triangle? was it sss, sas, asa, aas, or hl?

Atriangle with all sides of equal length is a/an triangle. a. right b. scalene c. equilateral d. isosceles

How is working backward similar to solving an equation?