Quran 20:25-28
رَبِّ اشْرَحْ لِي صَدْرِي وَيَسِّرْ لِي أَمْرِي وَاحْلُلْ عُقْدَةً مِّن لِّسَانِي يَفْقَهُوا قَوْلِي
Rabbi ishrah li sadri, wa yassir li amri, wahlul 'uqdatan min lisani, yafqahu qawli
My Lord, expand my chest for me, ease my task for me, and remove the impediment from my tongue so that they may understand my speech.
When to recite: Before starting to study or before entering an exam hall. This was Prophet Musa's (AS) dua when given a difficult task.