Fa laa uqsimu bi rabbi al-mashariqi wa al-magharibi inna laqadiruna
So I swear by the Lord of the easts and the west that surely We are able
The following is an excerpt of commentary on this ayah, from Tafsir Ibn Kathir:
the One Who created the heavens and the earth and made the east and the west. He is the One Who subjected the stars causing them to appear in the eastern parts of the sky and vanish in the western parts of it. The point of this statement is that the matter is not as the disbelievers claim: that there is no final return, no reckoning, no resurrection and no gathering. Rather all of this will occur and come to pass. There is no way of avoiding it. This is the reason that Allah has stated a negation at the beginning of this oath. This shows that He is swearing by a denial of their claim. This is a refutation of their false claim of rejecting the Day of Judgement. They already witnessed the greatness of Allah's power in what is more convincing than the Day of Judgement, that is the creation of the heavens and the earth, and the subjection of the creatures in them, the animals, the inanimate objects and the other types of creatures that exist