Given Numbers = 1, 2, 3, 4, 3, 2, 1
So we can form 7 digits numbers.
Here 1th , 3th , 5th , 7th are , Odd places which we have to fill odd numbers.
In Given question odd numbers are = 1, 3, 3, 1
Number of places to arrange them
We have 2th , 4th, 6th are even place.
Even numbers are = 2, 4 , 2
Number of ways to arrange them
Total number of ways = 3 × 6 = 18 ways.