3) A password consists of 6 characters. How many different passwords are possible if the password contains a) 2 lower case letters (a-z) followed by 4 digits (0-9)? b) 2 letters (upper or lower case) followed by 4 digits (0-9)? c) How about if each of the 6 characters can be an upper case letter (A-Z), a lower case letter (a-z), or a digit (0-9)