Where did you get that? The diagram doesn't appear to correspond with the text. You should presume that the diagram and text will have been checked and do correspond in real exams.
What you'd do for ray trace type questions do is draw a normal at the point the ray inside the prism meets the next surface and work out it's angle of incidence... It should be clear what to do based on your knowledge of snells law and critical angles.
Refractive index of air is 1.00 unless the question says otherwise.