## Discrete Mathematics and Its Applications, Seventh Edition

$26^6$
We are selecting a string consisting of six letters, each of which can be any of the $26$ letters, with repetition allowed. There are $26$ choices for the first letter, $26$ choices for the second letter, and so on. By the product rule on page 386, the total number of ways to select five elements is $26*26*26*26*26*26=26^6$.