The solution by Alan is correct only if positions of letters and digits are fixed:

for example, two letters go first and three digits follow in positions 3, 4 and 5.



                 But the post does not state it.



If, in opposite, the positions are not fixed (letters and digits can be in any positions),

then this number 3*3*6*6*6 = 1944 from the Alan's post should be multiplied by 

 =  = 10, so the final answer is  3*3*6*6*6*10 = 19440.