Answer:
Step-by-step explanation:
This problem has no specific right answer. I can give you two possibilities. If you get the first one wrong, try the second.
1 molecule of BaF2 has
1 Ba 1
2F 2
Total 3
3 molecules have 3 * 3 = 9 atoms
That's one answer
The other is the actual number of atoms in 9 units
1 unit has 6.02 * 10^23 atoms
9 units have 6.02 * 10^23 * 9 = 5.42*10^24
Since the question is unclear, you will have to choose.