For a) this is my work:

cos 2θ + cos θ = 0

2 cos^2θ - 1 + cos θ = 0

2cos^2θ + 1 cos θ = 1

1) cos θ = 1

2) 2cos θ + 1 = 1

cos θ = 0

I got those two equations above from factoring out the problem. However The answer in the back of the book says, 60, 180, 300 degrees. I know that what I got for an answer will not come out with those degrees. So can you help find what I did wrong? Thank you so much.