From the answer, let's assume that you can only use each number once.
So there are four slots.
_ _ _ _
For the first slot, you have a possibility of 9 numbers.
9 _ _ _
Now for the second slot, you have the possibility of 8 numbers.
9 8 _ _
Now for the third slot, you have the possibility of 7 numbers.
9 8 7 _
Now for the last slot, you have the possibility of 6 numbers.
9 8 7 6
Now multiply these numbers together and you should get 3,024.
