Call for the Batmobile and head toward the GCPD police station, where you have already been before. Enter the underground parking garage, stop the vehicle at the designated space and enter the station. You have to reach Poisonous Ivy's cell, so that Batman could convince her for cooperation. By the way, have a chat with officer Cash. Go back to the Batmobile with Ivy. While you are leaving the underground parking garage, Cash informs you of a huge Cobra tank that appeared in the area. You will have to figure out how to destroy it.