2. An arrow over a number makes no sense! Normally, an arrow over a letter indicates a vector but then it cannot be a number. I rather suspect that it is actually a "/" indicating ordinary division: $cos^{-1}(cos(4\pi/3))$. $4\pi/3$ is outside the natural range of arccosine, which is 0 to $\pi$ so $cos^{-1}(cos(4\pi/3))= 2\pi- 4\pi/3= 2\pi/3$.