Here's how to back-up and restore your bookmarks:
Step 1:
Open Chrome.
Click the three dots at the top right.
Select Bookmarks > Bookmarks Manager.
At the top left, click More (three dots) > Export Bookmarks.
Export the file to any convenient location on your Chromebook.
Step 2:
Now copy the file you've just created to any location of yout choice which ISN'T on your Chromebook. For example, send an email to yourself with the file as an attachment, so that the file will be on your email provider's server.
Step 3 (to be used AFTER restoring your Chromebook to its factory settings):
Retrieve the file from wherever you stored it. (i.e. if you emailed it to yourself, go to your Inbox and download the attachment to any convenient location on your Chromebook)
Step 4:
Open Chrome.
Click the three dots at the top right.
Select Bookmarks > Bookmark Manager.
At the top left, click Organise.
From the drop-down menu, select Import Bookmarks from HTML file.
Navigate to the location of the file that you've stored your bookmarks in and import it.
You'll then have all of your old bookmarks back again.