See the FileVault recovery key for your Mac in Passwords
When you protect the data on your Mac with FileVault, you get a recovery key during setup. If you forget the password for your Mac, you can reset the password by entering the recovery key.
Go to the Passwords app
on your Mac.Click All in the sidebar, then click the name of your Mac in the list.
If you don’t remember the name for your Mac, go to the System Settings app
, click General 
in the sidebar, then click About and look at Name.Hold the pointer over the recovery key to reveal it.
You can click the recovery key to copy it.
Note: You can see the recovery key for your Mac in the Passwords app on your iPhone or iPad where you’re signed in to the same Apple Account as your Mac.