Observe the figure given.
Let us complete the given paragraph:
It is given that E is the midpoint of DF. So, DE by the definition of midpoint.
As, midpoint divides the line segment into two equal halves.
Therefore, DE =EF by the segment congruence postulate. DE+EF = DF by the segment addition postulate and so DE+DE = DF by substitution.
Segment Addition Postulate states that given 2 points P and Q, a third point S lies on the line segment PQ if and only if the distances between the points satisfy the equation PS + SQ = PQ.
Simplifying gives 2DE = DF.