This a a one line command to enter in terminal. See
What is a terminal and how do I open and use it?
To use it, you would paste the entire command in the terminal (remember to use https):
wget -qO - https://deb.opera.com/archive.key | sudo apt-key add -
But of course, it is daunting just copying and pasting commands without knowing what they are doing, and having no instructions on how to undo their actions, so here is a basic breakdown of the commands:
wget downloads something from a server. See wget manual for Ubuntu 16.04.
| is a pipline, which takes the output of one command and runs it into the input of another
apt-key add adds a package key
So it basically downloads the key and then adds it in one command.
I tested the command and it should work.
Now to verify that it worked, run this command (from this answer):
This will list the keys added and the key from Opera should be listed on the bottom like this:
pub 1024D/30C18A2B 2012-10-29 [expires: 2014-10-29]
uid Opera Software Archive Automatic Signing Key 2013 <email@example.com>
sub 4096g/C528FCA9 2012-10-29 [expires: 2014-10-29]
The linked answer also shows that you can remove the key if needed, using:
sudo apt-key del 30C18A2B
30C18A2B being the key-id from the list.
After performing that command, and setting up the sources exactly like in your screen-shot, do:
sudo apt-get update
sudo apt-get install opera
(note there are some random warnings, but nothing that affects the install or software center operations)
And for the removal (just in case): What is the correct way to completely remove an application?
So in summary:
- Add repository
- Add key with
- Install in terminal with
- Search in dash