Why is #x= _3C_9# impossible to evaluate?
It's not impossible to evaluate: it's just 0.
The best way to think of
In your case, that would mean "how many ways can I choose 9 things from 3 things?" If I only have 3 things, there is no way I can choose 9 things. Hence, there are 0 possible ways to do it.
If you wanted to consider