Students often struggle with deciding whether the problem required permutations or combinations.
The key is to decide whether order must be considered or not when counting the possibilities. If order matters and selections may not be repeated, then you should use permutations (via the permutation formula). If order does not matter and selections may not be repeated, then you should use combinations (via the combinations formula). If order matters and selection may be repeated, then you have permutations but you will need a slot diagram to count the possibilities