Hi Cindy! Here's how I did it:
I placed the text (Done? Click Submit) on the slide and set it's initial state to hidden.
I set the items in my drag-and-drop to be revealed one at a time, and set the last item to be the "sunhat". (you can choose which order the items are revealed in, if you have them revealed one at a time)
I then created a Slide Trigger that changes the state of the Done? text from Hidden to Normal when the "Sunhat" graphic is dropped on either the garbage drop target OR over the beach bag drop target. Since the sunhat is always the last item to be revealed, I know that after it's been dropped, they are done the interaction and its safe to display the text.
You can take a closer look at it in the file which is available for download here:
https://community.articulate.com/e-learning-examples/storyline-2-sun-safety-drag-and-drop I hope this explains it! Let me know if you have any further questions!