Question 395876
Hey, 
So for this one you need to think about what the possibilities are for each letter:
For the first one, there are 26 possibilities, for the second there are 25 which are all the letters except the first one, and for the third there are 24 which are all the letters except for the first and second. Therefore:
26 x 25 x 24 = 15600
I hope this helps!