I recently got myself a HDMI cable and I've been attempting to mirror my screen from my laptop to the TV. However, the only option Zorin OS 15.3 Lite gives me is to use the TV as a second display, instead of allowing me to directly mirror my laptop content to the TV. The mirror display check box seems to be permanently disabled and I'm not sure if this is supposed to be the case. If you need anymore guidance, I'll be available 
Thanks in advance!
In Displays, is your secondary display connected via HDMI set to enabled? Once you do that, the box shouldn't be greyed out.
What is the computer you are using, and what is the secondary display your trying to connect to mirror too? I am curious to know if both your screens support the same resolution.
I am using a HP Stream 14 with max resolution of 1366x768. I am unsure about the TV model, so I will check again and see. I'm confused though, because I don't see any "Enabled" option.
This is the Displays settings window, this is where you turn displays on/off, and where you choose display options. If you have your TV plugged into your notebook, your notebook should be able to see your TV.
Your laptop is the box in RED. See the switch that shows ON? That means your display is enabled. Now, if you click on the GREEN box, which should be your TV display, does that switch say ON as well? Now I remember you said the mirror box was grayed out, I just wanted to make sure the display was at least enabled, hence switched on in Displays options.
Now, here's the thing, your notebook is using a common resolution of 1366x768 for a small 14" screen. That however, is not a common resolution for a TV, common resolutions for TV's are 1280 x 720, 1920 x 1080, 3850 x 2160. In order to mirror the TV, it has to support that lower resolution, and I doubt that it does.
This could also be the reason why the box is grayed out. Which means, you either have the option to extend the desktop to your TV instead, or, switch to using the TV as a main display only. Which is what I am doing BTW, I don't use my notebook screen.
My laptop supports this, maybe I could mirror it if I set the TV and laptop resolution it would work?
Plus. both displays are enabled
Yes you could totally try, it certainly wouldn't hurt! It would at least let you know if you can get this thing working if you can match resolutions!
Unfortunately, no matter what resolution I change it to, the mirror displays button stays disabled. I'm beginning to doubt whether my TV is compatible or something. It's an old LG TV, which according to Zorin OS, is 72 inches (screen size).
Will this terminal command mirror your display?
xrandr --output (Put your Secondary Display ID Here) --auto --same-as (Put your Primary Display ID Here)
You can get your display ID's with
xrandr --query
For example, if you have two HDMI out ID'd as HDMI-0 and HDMI-1 with HDMI-0 as primary:
xrandr --output HDMI-1 --auto --same-as HDMI-0
