Hi @amanda34 ,
Things to try to narrow down what is the probable cause of the problem.
a). Start the laptop in [https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/|safe mode] and check if the problem is still there or not. The screen will look different because only generic Windows drivers are loaded and not drivers specific to the hardware in the laptop. This is only to check if it the problem is there.
''If it is not there,'' then it is a video driver problem. Check that you have the latest [https://www.asus.com/us/ROG-Republic-Of-Gamers/ROG-G751JT/HelpDesk_Download/|Asus Win 10 VGA drivers] installed.
''b). If the problem is still there in safe mode,'' connect an external monitor to the laptop’s HDMI port and check if the problem is also on its display or not. Even a TV can be used as an external monitor as long as it has HDMI inputs and that you have a compatible HDMI cable to connect the two together. Don’t forget to operate Fn + F8 (both together) to toggle the display between laptop screen only, external display only or both on together.
c). ''If also showing on the external monitor screen,'' then there is a motherboard problem
d). ''If only on the laptop screen,'' then the laptop would need to be opened and the video cable connection between the motherboard and the LCD panel would need to be inspected at both ends (motherboard and screen) to ensure that it had a secure connection. If it appears to be OK then as a DIY repair all you could do is to temporarily replace the LCD panel to check if it is the cable or the LCD panel.
Here’s a link to a [https://www.asusparts.eu/en/asus/notebook/g-series/g751jt?ProductCategory=Display|supplier] of parts for your model laptop. If you click on the ''Display'' checkbox you will find the Asus part numbers for the screens that are compatible with the model. If you then search online using the part number ''only, y''ou will get results for other suppliers that may suit you better. Usually the part number is printed somewhere on the back of the panel if you wish to verify that you’re getting the correct one.