Who is asking: Parent
Imagine that you are going to fill in the field in all possible ways.
Since there are 26 letters and 10 digits you have a total of 36 characters to choose from for each position. Starting from the left you can fill in the first position in 36 possible ways. For each of the 36 possibilities for the first character there are 36 possibilities for the second character. Thus there are 36 times 36 or 362 ways to fill in the first two characters.
For each of the 362 ways to fill in the first two characters there are 36 ways to fill in the third character. Hence there are 363 ways to fill in the first three characters.
Continuing in this fashion you can see that there are 366 = 2 176 782 336 ways to fill in all 6 characters.Cheers,