I have nine books and have room to display only four of them at a time on my bookself. How many different ways could I do this?

So to do this on paper I'd usually write out four blanks. One to represent each spot on the shelf. Then think about how many options you have for the first spot, 9. Now that you've used one, you second spot can only have 8 options. Since we've used two the third spot only has 7 options now. Finally the fourth spot has only 6 options. To get the answer we not multiply these four numbers

9x8x7x6 = 3,024