Explanation:
"clipboard" contains 9 different letters (no repetitions).
we are asked for the permutations, so the sequence (or order) of the selected letters matters (e.g. "lod" is different to "dol").
P(9, 3) = 9!/(9-3)! = 9!/6! = 9×8×7 = 504
504 different 3-letter permutations can be formed.