The proper answer should be to speak with the person you bought it from and if they don't know the key, you will never know if you "own" stolen property.
The quick fix is to power it down then start it up again, this time holding down the VolumeUp button whilst pressing the Power button.
When you see a menu of options, using the VolumeUp and VolumeDown buttons to navigate to Factory Reset and press the Power button accept that option.
All the personal data the real owner wanted to protect will have gone along with any changes made since the device was new. You will have a clean tablet.