So at first we'll say, well look, I have 9 slots in my hand, 1, 2, 3, 4, 5, 6, 7, 8, 9. I'm going to pick 9 cards for my hand. And so for the very first card, how many possible cards can I pick from? Well, there's 36 unique cards, so for that first slot, there's 36. But then that's now part of my hand. Now for the second slot, how many will there be left to pick from? Well, I've already picked 1, so there'll only be 35 to pick from, and then for the third slot, 34, and then it just keeps going, then 33 to pick from, 32, 31, 30, 29, and 28.