Here is a guide on how to hard reset original Samsung GT-P3100 Android Tablet running on V4.1.2. This is just a very common key combination trick.
1. Press Volume Up and Power On key at the same time.
2. Un-press when you see Samsung logo. You will see the Android icon then you will be directed to Android Utility.
3. Select "wipe data/factory reset", press Power On key to do the process.
4. Select "delete all user data", press Power On key to do the process.
5. Finally Reboot. Smile.
