Unlock bootloader (WARNING - Factory Reset!)  
assuming you have already installed usb-drivers and [fastboot.exe][1]

- press `Power` button until your tablet is off
- press and hold together `Volume Down` + `Power` button
- Press `Volume Up` to enter Download mode  
- connect USB cable to PC
- then type from `cmd.exe` in fastboot directory

    `fastboot oem unlock`  


- if your bootloader is unlocked you can delete cache from fastboot

      `fastboot erase cache`  
      `fastboot format cache`

**TWRP is available for your device, you can boot it and wipe cache**  

 - download [twrp-2.8.5.0-p5100.tar][2]   
 - unpack recovery.img with 7-zip to fastboot directory
 - boot into twrp without flashing it

    `fastboot boot recovery.img`  

 - check the TWRP Menu / Advanced / Wipe


  [1]: https://www.xda-developers.com/google-releases-separate-adb-and-fastboot-binary-downloads
  [2]: https://dl.twrp.me/p5100/twrp-2.8.5.0-p5100.tar