Hey,

Each corner atom of hexagonal face is shared by 6 unit cells, i.e. they contribute 1/6th of the mass. There are two such hexagonal faces i.e. 12 such atoms.

Thus contribution of corner atoms=2×6×1/6= 2

The atom at the centre of the hexagonal faces is shared by two cells each.

If your doubt not get cleared you can ask your query again.