Given only this graph of a certain cosine function, how can I determine what it's "c" is? (click to see graph)

Question

Although my answer key says $c$ is supposedly $\frac{2\pi}{3}$, I actually get $c = \frac{4\pi}{3}$

Thus, $y$ should equal $\frac{5}{2}cos(\frac{\pi(x)}{2}-\frac{4\pi}{3})-0.5$ , right?