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26 letters in the alphabet.
first 2 must be lower case letters so 26*26 are all possibilities.
remaining 4 can be lower case letters or digits.
10 digits from 0 to 9 makes 26 + 10 = 36 possibilities for each position.
total should be:
26^2 * 36^4 = 1.135420416 * 10^9 possibilities.
that's equivalent to 1,135,420,416 possibilities (1 trillion, 135 billion, 420 million, 416).
to see how this works, use much smaller numbers.
assume 5 positions total
assume first 3 positions can each be one of 2 letters only (a or b).
assume next 2 positions can each be one of 2 letters only plus 1 number (a or b or 1).
possibilities are 2^3 * 3^2 = 8 * 9 = 72
possibilities for the first 3 positions would be:
total of 8
possibilities for the next 2 positions would be:
total of 9
when you combine the first 3 positions with the last 2 positions, you can get a total of 8 * 9 different possibilities.
example of 2 of the possibilities when you combine the first 3 positions and the last 2 positions would be:
6 more of these gives you the 8 * 9 total possibilities.
same principle applies to the larger set of numbers in your problem.