What is the equation for Snell's law?

where
#n_1# is the index of refraction of the first substance that light travels through
#theta_1# is the angle of incidence
#n_2# is the index of refraction of the second substance that light travels through
#theta_2# is the angle of refraction

Then you need to isolate your unknown.

Example:

Light in air (#n_1# = 1.0003) is incident upon a block of crown glass (#n_2# = 1.52) at an angle of #35.0^o#. What is the angle of refraction?

#n_1# sin #theta_1# = #n_2# sin #theta_2#
(Isolate sin #theta_2# by dividing both sides by #n_2#)
#n_1# sin #theta_1# / #n_2# = sin #theta_2#
(1.0003) (sin 35.0) / 1.52 = sin #theta_2#
0.5737 / 1.52 = sin #theta_2#
0.3775 = sin #theta_2#
(To find #theta_2#, you need to find the inverse sine of this value. On your calculator, press the 2nd function key, then the sine key.)
#22.2^o# = #theta_2#