Need help please: Total Internal reflection?

The drawing shows a crystalline slab (refractive index 2.090) with a rectangular cross section. A ray of light strikes the slab at an incident angle of 1 = 41.0�, enters the slab, and travels to point P. This slab is surrounded by a fluid with a refractive index n. What is the maximum value of n such that total internal reflection occurs at point P?