Hard Reset SAMSUNG T550 Galaxy Tab A 9.7"

First method:

1. At the very beginning switchoff the tablet by using the Power key.
2. After that, press and hold together Volume Up + Home + Power button for a few seconds.
3. Release held keys when the Samsung Logo appears on the screen.
4. Select "wipe data / factory reset" by using Volume buttons to navigate and Power button to confirm.
5. In the next step choose "Yes -- delete all user data" to confirm whole operation.
6. When the Recovery Mode appears again select "reboot system now".
7. Good job! The hard reset has just been performed.

Second method:

1. If the display is off, press the Power button to turn it back on.
2. Afterwards go to Settings and then to Backup&Reset.
3. Tap Factory Reset Device and Reset Devive. 
4. Choose Erase Everything if you are sure you want to clear all your data. 
5. Select OK to confirm the whole operation.
6. Well done! The hard reset is done.

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.