This problem can be solved using the concepts of permutation.
Zzyzx is the word. It has five letters. It asked as to how many five-letter arrangements of the letters in the English alphabet follow Zzyzx alphabetically.
Take note: alphabetically
The two remaining five-letter after Zzyzx are Zzyzy and Zzyzz.
The remaining arrangements are of the form Zzz _ _.
There are 26 × 26 = 676 ways to fill in the last two letters, so following Zzyzx alphabetically, there are 2 + 676 = 678 arrangements.