Letters in the alphabet = 26
possible digits (0 to 9) = 10
Since it consists of 3 digits and 3 numbers:
The first digit has 10 possibilities, also the second and third digit.
So:
10x10x10
Same for the letters:
26 x 26x26
Alltogheter:
10x10x10x26x26x26 = 17,576,000