"If the worshipper doubts [whether he has performed] three or four rak'ahs, he must base his action upon certainty (al-yaqeen) – which is the lesser number (three). He then completes the prayer based on that certainty, performs the fourth rak'ah, and then performs the prostration of forgetfulness (sajdatay al-sahw) before the Salam (Tahiyyah).
If you provide more specific information about the content on page 89 (such as the topic, any specific rulings, or terms you're unsure about), I could offer more direct assistance. sharh hanafiyah page 89