If you look at the man page for `touch`, you'll see that it takes a filename as an argument. So what you need is:
touch ../cambridge/library/dir_practice
Since you're new to this, let's break this down. A relative pathname is a path specified relative to your current location. Contrast this with an absolute pathname, which starts from the root (/).
So you start at `sample_dir/oxford`. The first `..` takes you back to the `sample_dir` directory. Then, you want to go to the `library` directory, which is inside the `cambridge` directory. Inside the `library` directory, you want to create the file named `dir_practice`.
Put all that together, and you get `../` \+ `cambridge/library/` \+ `dir_practice`. Together, that's `../cambridge/library/dir_practice`. This is the argument we pass to the `touch` command.