(The following are all vectors)

EF=EA+AB+BF EF=EC+CD+DF

2EF=(EA+EC)+AB+CD+(BF+DF)

E and f are the midpoint of diagonal AC and BD, respectively.

So EA+EC=0 BF+DF=0.

2EF=AB+CD

EF=(AB+CD)/2=3a-2.5b+3c