There are gobs of additional applications waiting for you to install them on the Google Nexus 9 tablet that cannot be installed until you grant yourself the full control over the operating system that allows you to install them. Similarly, there are also a great many apps already on your device that will not uninstall unless you get access to the same root user account that allow you to have the full control over the operating system.
One of the apps you can install that requires root access and will remove the system apps already on your device that will not uninstall without root access is the Titanium Backup app. The Titanium Backup app is mainly used when people want to take full backups, but it also can remove any of the apps that are embedded into the system partition—the place that you cannot get to without access to the root user account which is why phone carrier networks install them there.
- You need to have the Google Nexus 9 WiFi tablet to use this guide otherwise you will need different custom recovery image.
- You need to have a computer that is running on a version of the Windows operating system to use this guide or else the ADB in our guide will not run.
BEFORE WE BEGIN
- You need to have the Developer Options menu unlocked on the Google Nexus 9 tablet, and the USB Debugging Mode enabled before you can use this guide and make the changes to the Android software that is required for the rooting to work.
- Install the ADB Drivers  on the computer before you follow the guide for setting up the ADB so that your tablet can connect to the computer and use ADB when the time comes for that.
HOW TO ROOT GOOGLE NEXUS 9 TABLETS RUNNING ON ANDROID 7.0 NOUGAT (NRD91D FIRMWARE)
- Follow the guide for setting up ADB on the Windows  operating system and then come back and continue with this guide after you have completed that task.
- Download the TWRP image  that works for the Android 7.0 Nougat on the computer for it to be flashed with ADB.
- Download the SuperSU  that works for Android 7.0 Nougat that you will be flashing from the custom recovery after it is installed.
- Boot the Google Nexus 9 into the bootloader mode and then connect it to the computer with the USB cable.
- Open the Downloads folder and extract the TWRP file to the downloads folder and then copy everything from that TWRP file over to the ADB folder, so you have it all available to you int the ADB directory.
- Copy the SuperSU from the Downloads folder to the internal storage SD card folder on the computer for the tablet that always appears when you have it connected to the computer with the USB cable.
- From inside that ADB and TWRP folder, hold the Shift key down and right-click the background where it white and there are no files, and then choose to open a new command window from the menu.
- Type the first command “adb reboot bootloader” if you do not have it in bootloader mode yet or “fastboot flash recovery twrp-3.0.2-0-flounder.img” to flash the custom recovery after you have it in bootloader mode.
- Type the command “adb reboot” and then as soon as your Google Nexus 9 reboots, you need to hold down the hardware button combination for the Recovery Mode.
- Since you just flashed the custom recovery over the stock recovery partition, you can expect to see the custom recovery now booting on the tablet’s display.
- Tap on the Install button from the Recovery menu and then follow the guidelines to browse the SD card and install the SuperSU package that you transferred there earlier.
- Choose the Reboot option followed by System after the flashing is done and the tablet then reboots back into the normal mode and is rooted with a custom recovery installed.
In conclusion, that is how to root Google Nexus 9 running on Android 7.0 Nougat NRD91D using a custom recovery image and flashing Chainfire’s SuperSU. You can see the SuperSU app available from the app drawer once the tablet does reboot, but you do not need to enter it before you start installing the root apps. It is already enabled correctly and ready to grant the rooting rights over the operating system as soon as you are ready to download your apps that need root access before they will run.