How many different strings of 5 lower case letters are there

Question

How many different strings of 5 lower case letters are there? Assume that letters may repeat and the string must contain at least one letter a?
I tried this:
5×26^5=5×26 ×26 ×26 ×26=2684880

Answer 1

$(26)^5 - (25)^5.$

First term is total # of 5 character strings, where each character has 26 choices.

Second term is total # of 5 character strings, where each character has only 25 choices, re no "a" is used.

The difference between the two is the # of 5 character strings with at least 1 "a".