I came across a word problem that I can't seem to puzzle through. I know the answer to it, but I can not seem to find out how to get to it.

"A block is set in motion hanging from a spring and oscillates about it's resting position x=0, according to the function . For what values of t is the block at it's resting position x=0."

This is how I've worked it so far.

This answer, unfortunately, is not correct. I know that the correct answer is 75 degrees, but I have no clue how to get to that answer.

Any help would be appreciated. I've been puzzling over this question for a while now.