Find candidate keys given functional dependencies
Assuming that the three dependencies are a cover of the dependencies of the relation schema R, to find all the candidate keys we could start from a canonical cover of the FDs, for instance the following one:

{ A B C → D
  A B C → F
  D E F → A
  D E F → C
  D → B }


From these dependencies we can see that E must belong to all the candidate keys, since it does not appear in any right part. 

We can start now from the left hand parts of the dependencies to see if their closure contains all the attributes:

ABC+ = ABCDF


by adding E (that must be present in every candidate key) we discover the first candidate key, since ABCE+ = ABCDEF.

Then from DEF:

DEF+ = ACBDEF


so this is already a candidate key.

Finally, starting from  D we found that:

D+ = DB


trying again to add E, we find:

DE+ = BDE


we can now try to add A:

ADE+ = ABDE


which is still not a candidate key, and finally adding C we find the last candidate key:

ACDE+ = ABCDEF