Help with this physics problem?

[penz]

A light ray of wavelength 589 nm is incident at an angle ? on the top surface of a block of polystyrene as shown in Figure P25.53. (a) Find the maximum value of ? for which the refracted ray undergoes total internal reflection at the left vertical face of the block. (b) Repeat the calculation for the case in which the polystyrene block is immersed in water. (c) What happens if the block is immersed in carbon disulfide?

There is a diagram which can be found in this website # 53 (http://www.piercecollege.edu/faculty/meyermd/Physics%2067/P67%20Homework/Chapter%2025%20HW%20PDF.pdf )

For air, tried this where n1=1.00, and for the polystyrenen2= 1.49 and did [n1/n2]sin^-1=? and got 42 degrees. Can some one check this problem, if something needs to be changed, please show the right answer, and how it was done, thank you!