To comment or post a followup question to a Q&A post, please click within the text box beneath the post that reads "Compose a new followup discussion."

When a new followup is posted, it will automatically be set to Unresolved, and an icon reading 1 unresolved followup will display beneath the post in your Q&A feed, as seen below:

You and your classmates can resolve a followup by selecting Resolved just above the followup, as seen below. Should you have additional questions, you can always mark the followup back to Unresolved again. 

To reply to a followup, simply type your response into the text box beneath the followup itself: