I, for the life of me, cannot figure out this equation:

Question

The index of refraction for a particular type of glass is 1.52 and the index of refraction for air is approximately 1.00. A ray of light at a particular frequency enters a block of glass from air with an angle of incidence of 70 degrees. What is the angle of refraction O^r?

Please walk me through your steps so I can really see how you worked through this one-I've been working on it for about two days now...
thanks