Critical Numbers Problems

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Okay so I found the critical number no problem, it being cos x=-1/2, but on my answer sheet it says that the critical numbers are 2pi/3 or 4pi/3. How are they getting these numbers?

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$$\cos x=-\dfrac{1}{2}\\ \implies \cos^{-1}(\cos x)=x=\cos^{-1}\left(-\dfrac{1}{2}\right)=\dfrac{2\pi}{3},\dfrac{4\pi}{3}\text{ on graphing.}$$