Question

Feeling flustered.. The question is attached below and i've also included my working out but i'm not sure why my answer differs from the answer in the book?
Question:
http://prntscr.com/5rxedu

My working out
http://prntscr.com/5rxf5u

Answer 1

The general solution to \[2x = \cos ^{-1} (\frac{1}{2}) = \pm\frac{\pi}{3} + 2n \pi\]
Hence, \[x = \pm\frac{\pi}{6} + n \pi\]
This is evident from the graph actually. 2x squeezes the graph.
Understood?