how many letters can be placed in each of the first three blanks? there are 26 letters right? therefore, you can choose any of the 26 for the first blank, 26 for the second, and 26 for the third..
for the next 3 blanks, how many choices do you have? 10 each (0,1,...,9)
therefore, the number cars of possible is the product..
26 x 26 x 16 x 10 x 10 x 10
b) if a letter has been chosen to one of the first three blanks, (whether the first, second or third), you would have no choice to other 2 blanks of the first three.. so you would have
26 x 1 x 1 x 10 x 10 x 10