What is the difficulty flagged by “wa-hādhā mushkil”?

The Companions’ Disputes, Ijtihād, and the Rightly Guided Caliphate

• The use of “caliph” for rulers long after thirty years
• The reasons for ʿUthmān’s assassination
• The location of battles during ʿAlī’s reign
• The exact months and days of al-Ḥasan’s rule