Tell me if my first answer is right and the rest of mine will be. .)
answer to a.) There would have been 10 possibilities for each number and 26 possibilities for each letter to begin with. The six character listen plate would have : 10*10*10*26*26*26 combinations or 17,576,000. There are now 36 possibilities for each character which means 36^6 or 2,176,782,336. The second one subtracted by the first one makes for 2,159,206,336 more possibilities.
This dosent seem to go along with the lesson in the book(permutations) thats why im wondering