How to perform a Factory Reset on SGPT121US Sony Xperia Tablet

To get into factory reset menu by hardware level  follow  instructions below:

1. Power on the Tablet (press the power button for like 1.8 Seconds, then release it)
2. Immediately Press and hold the Volume + for three seconds.
3. Without releasing Volume +, Press the power button for one second.

The amount of time required to complete this task  will be 5.8 to  6 seconds  from step 1 to 3, then with the volume+ and volume- navigate on the menu, and to confirm any action press the power button.