a. Triangle ABC is an equilateral triangle because all three angles are equal to 60° each. Therefore, all three sides would also be equal to each other.
Thus, since one of the sides, BC = 6, therefore:
AB = BC = 6
b. The markings indicate that two sides of ∆EDF (ED & EF) are equal in length. This means that ∆EDF is an isosceles having two equal base angles (<F and <D).
