Question 1090292
Looking at the shelf, you see 3 positions:
left, middle and right.
You have {{{7}}} choices for a book to place in the left position.
For each of those choices, you would have {{{6}}} books left to choose from for the book to place in the middle position.
So far, that would give you {{{7*6}}} different possibilities.
For each of those, there would be {{{5}}} different ways to choose the book to place on the right position.
All in all, there are {{{7*6*5=210}}} different arrangements for 3 books chosen from a set 7 books.