2177280 serial numbers can be formed.
For the first part, we can have
9(8)(7)(6) = 3024 possibilities. This is because there are no repeats; we can have 9 for the first letter, 8 for the second, and so on.
For the numeric part, we can have
10(9)(8) = 720 possibilities. This is because there are no repeats; we can have 10 for the first number, 9 for the second, and so on.
Together, this gives us 3024(720) = 2177280.