# Why is x= _3C_9 impossible to evaluate?

The best way to think of ${\setminus}_{n} {C}_{r}$ is as "n choose r", or "how many ways can I choose r things from n things?"
If you wanted to consider ${\setminus}_{9} {C}_{3}$, we can easily calculate that:
\ _9C_3 = (9!)/(3!6!) = (9*8*7)/(3*2*1) = 3 * 4 * 7 = 84