A city distributes vehicle identification stickers using a combination of letters followed by a combination of digits, each of which may be used more than once. Determine the number of possible stickers under the given circumstances.
a) Using 6 letters and 2 digits, how many stickers are possible?
b) Using 2 letters and 6 digits, how many stickers are possible?
c) Adding 1 letter before the 6 letters and 2 digits, how many stickers are possible?