You can take a look at this thread. There is an instruction to install the kelebek ppa.
You would know if you have added it. You can find the added ppas at Software &Updates at Other Software.
Proprietary Nvidia driver 390 don't work with the normal 6.8 kernel of Zorin 17.3. It only works with kernel 5.15 or when you add kelebek333 ppa. Otherwise you can use nouveau driver for your nvidia card when you are no gamer and don't need the performance.