What is the missing step in this proof? (see the entire proof.)

statement: slope of df = slope of bc
reason: definition of slope

statement: slope of df = slope of bc
reason: transitive property of equality

statement: df = bc
reason: corresponding sides of congruent triangles are congruent.

statement: δadf≅δabc
reason: asa
done
given: δabc (view diagram )
prove: a midsegment of δabc is parallel to a side of δabc.

statement reason
(steps 1−5)
6. ac de definition of parallel lines
7. let f be the midpoint of ac. defining a midpoint
8. f=(x1+x32,y1+y32) definition of midpoint
9. slope of df = y1+y32−y1+y22x1+x32−x1+x22 = y3−y2x3−x2
slope of bc = y3−y2x3−x2 definition of slope
10.
11. df bc definition of parallel lines
12. similarly, ef is parallel to ab. steps similar to steps 1−11