1.- AC congruent with DF Given
2.- AB congruent with DE Given
3.- mAC is congruent with mDF Addition property of equality
4.- mAB=mDE Definition of congruent segments
5.- mAB+mBC=mAB (should be mAC) Segment addition postulate
6.- mDE+mEF=mDF Segment addition postulate
7.- mAB+mBC=mDE+mEF Transitive property
8.- mAB+mBC=mAB+mEF Subtitution property
9.- mBC= mEF Subtraction property of equality
10.- BC congruent with EF Definition of congruent segments
