Question 7932
Now the formula you would use is {{{nPx=n!/(n-x)!}}}.  This reads, (the number of combinations of n things taken r times.) 

If there are a number of successive choices to make and the choices are affected by the previous choices (dependent upon order), then permutations are involved.

Now you did not state how many times that a letter is taken out of entire letters.  Are they taken out 2 at a time, 1 at a times. You have no (r) for the formula. 

Number of choices for the first letter 9 x
Number of choices for the second letter 8
Number of choices for third letter 7
Number of choices for the fourth letter 6
Number of choices for the fifth letter 5
Number of choices for the sixth letter 4
Number of choices for the seventh letter 3
Number of choices for the eighth letter 2
Number of choices for the ninth letter 1

9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 = 362,880 This is the number of possible combinations you can make with the letters A, B, C, D, E, F, G, H, I