I'm using GMKtec NucBoxG3 Plus PC with Zorin 17.3 Pro. The pc has 2 hdmi port. I connected 2 monitors on it. Both monitor are working but not in extended mode. I want to extend them but there is no option. In the Display Setting it only see one "unknown monitor".
Any help?
Thank you for posting your model, I understand now its one of those space saving Nuk PC's. It would be helpful to get more information. Please install Screenfetch in terminal with the following command...
sudo apt-get install screenfetch
Once installed, then just type the word...
screenfetch
Now, take a screenshot of the output you get, and post your screenshot in your reply. For best results, have the terminal window enlarged enough to see all the information.
Here's mine for example...
Welcome to the Forum!
Check if in Settings>About Your System runs in Wayland or X11. When it runs in Wayland, You could try it with switching to X11/Xorg.
To do that, go to the Login Screen (not the Lock Screen). Simply reboot for that. On the Login Screen click on Your Profile so that the Password Field appears. It has to be appeared. When it is appeared, You should see in the bottom right Corner a Gear Icon. Click on it and choose the Option ''Zorin Desktop on xorg'' and then log in and take again a Look in the Display Settings.
Thx for the info. I did it but it stay the same. Both monitors are working in mirrored mode. I need to put them in extended mode. Neither in Wayland or X11.
This is good. Wondered how to do that. Learn a lot on here. Wondering... how give pc a clean up like windows with ccleaner and wisecare etc. Used this - sudo apt update, apt list --upgradable, sudo apt upgrade, sudo apt autoremove, sudo apt clean, sudo rm -rf /tmp/*
Install Stacer...
Stacer is great for showing your computer's resource usage.
Stacer also has a cleaner built in, check it out!
You don't need to worry about defragging with a Linux GPT file system however, thats for Windows NTFS only.
Perhaps xrandr or arandr (that is a gui for xrandr) can help you?
To use this programs you'll have to switch to Xorg before as described by @Ponce-De-Leon.
There is also BleachBit or Ubuntu Cleaner.
Ubuntu Cleaner is a lot like CCleaner, as well as very safe and easy to use.
cool. thanks. take it that it's safe to select all 
I haven't ran my cleaning procedure in awhile, I just accomplished 2GB!
I also threw in a little BleachBit, the files were starting to get a bit stained lol.
