Chapter 11 - Further Topics in Algebra - 11.6 Basics of Counting Theory - 11.6 Exercises - Page 1060: 6

$15,600$

Order of the letters matter - Permutations. We are taking r=3 from n=26 letters, $P(n,r)=\displaystyle \frac{n!}{(n-r)!}$. $P(26,3)=\displaystyle \frac{26!}{(26-3)!}=\frac{26\times 25\times 24\times 23!}{23!}\\=26\times 25\times 24=15,600$