«وَأَنَّى تُشْرِقُ شَمْسٌ مَعَ غَيْمٍ؟»
How can the sun shine clearly while cloud covers it?
The sun can stand for the visibility of justice, and cloud for whatever obscures it. The speaker is not supplying weather information; he gives a vivid image for the difficulty of expecting perfect fairness under oppressive conditions.
A neighbouring question asks how justice could be produced from mistreatment. Both are rhetorical questions making the same argument: pressure and humiliation cannot easily generate an equal relationship.