How do you write the number 26 in numeric form?
26 or XXVI in Roman
How many possible alpha numeric combinations are there using all letters for 6 characters repeating alpha numeric?
Decide what symbols you want to allow. For example: 26 uppercase letters + 26 lowercase letters + 10 digits = 62. Or less, if you don't want to distinguish uppercase and lowercase; or more, if you want to allow certain special symbols such as spaces, hyphens, etc. Raise this number to the sixth power, if you use 6 characters.