Permutation and Combination Question

think of this as an arrangement problem -

1. digits that can be used 1-9.
2. no. of ways in which you can pick up a digit = 9, correspondingly you can pick up second digit to be used in number formation in 9 ways. ie. total ways= 9.8
3. for each way, there can be three 3-digit numbers. (for ex. if two digits picked are 1,2 resp., possible numbers will be 122, 212, 221)
4. so total possible such numbers = 72.3=216