it'd be 30 yard line in this case.
Basically, the ball placement is following:
If the kickoff hits end zone and downed or goes out of the back of end zone, the ball is placed in 30 yard line.
If the kickoff hits landing zone (goal line to 20yard line), bounces to end zone, and downed for touchback, the ball is placed in 20 yard line.
If the kickoff hits landing zone and stays in the landing zone, receiving team must return it.
If the kickoff is short of landing zone, the ball is spotted at 40 yard line.