you are only wasting my time if you're letting me do your homework for you, otherwise, if you're actually trying (and learning) i am more than happy to help.

there are two things you must consider:

and

is not a solution to either of those equations. and by the way, we think of angles to be in radians when doing calculus, unless otherwise stated.

think of the graph of cosine, think of the special angles and think of the quadrants in which the reference angles of the special angles appear and try again