break things down to sine and cosine. use your trig identities.

you have:

Note that if we have a solution. So find all the where is any odd integer is one set of solutions.

Now assume , we can divide by it to get

Now change out to something more convenient and continue

(instead of this thing about considering whether cosine is zero or not, the standard way would be to bring the cosine on the right to the left, factor it out and set each factor to zero)