On this page of our game guide to Batman: Arkham Knight you can find a description of another part of the story - meeting with Ivy. She resides at the GCPD lockup.
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.