Return a robot to its start position?
You can read the tacho count and rotate a negative amount of degrees. Or you may want to use the Navigation class and create a path while you move, then you just need to follow the path back.

But unless the robot is perfectly calibrated it will fail miserably. I explain it all here http://thetechnicgear.com/2014/06/howto-calibrate-differential-drive-robot/