a By the Converse of Base Angles Theorem we know that if two angles in a triangle are congruent, then the sides opposite these angles are congruent as well. Therefore, the statement we can make is that ED≅EF.
b In Part A we established that ED≅EF. Therefore, if DE=9mm it must be that EF=9mm as well. Also, since we know that the triangle's base angles are 54^(∘), then the vertex angle must be 180^(∘) - 2(54^(∘))=72^(∘). Let's add this information to the diagram.
Since we know the opposite angle to DF and at least one other side length and its opposite angle, we can use the Law of Sines to find the unknown side.
