this can be solve using snell's law. snell's law equation is :
N1 / N2 = sin a2 / sin a1
where N1 is the index of  refraction of the air which is equal to 1
N2 is the index of refraction of the medium
a2 is the angle of refraction
a1 is the incident angle
subsitute the given values
1 / N2 = 0.173 / 0.217
N2 = 1 ( 0.217 / 0.173)
N2 = 1.25 is the index of refraction
