#### Answer

1. Given
2. The midpoint of a segment is equidistant from the two ends of the segment.
3. Segment-Addition Postulate
4. Substitution Property of Equality
5. Substitution or Distribution Property of Equality
6. Division Property of Equality

#### Work Step by Step

1. Given
2. The midpoint of a segment is equidistant from the two ends of the segment.
3. The sum of the two pieces ($\overline{DE}, \overline{EF}$) equals to $\overline{DF}$
4. We can replace $\overline{EF}$ with $\overline{DE}$ because thy are egal.
5. DE + DE = $2\times DE$
6. We divide the equality by 2 to prove the statement.