As far as I can tell, this is a geometry problem that can be done by appropriately integrating over solid angles. The molecules are traveling with the same average velocities in all directions. Anyway, integrating the over all solid angles for a hemisphere to calculate the flux normal to a plane oriented perpendicular to the x axis is the way I would approach this problem. But, there may be simpler ways.The question is in the image file. I am stuck, I don't know how to start. Can someone guide me please?