I'm not great at math but I'll try my best.
I think we should find all the 6 digit numbers, and then find the number of them in decreasing order.
The number of 6 digit numbers is 8^6.
The number of digits in decreasing order is just the number of digits possible.
With the digits 1234356, there is only 1 way of putting it in decreased order.
So the number of ways possible where 123 is the same as 321 or 213.
I hope this helped. :))