Chapter 4 - Section 4.5 - Graphs of Sine and Cosine Functions - Exercise Set - Page 596: 41

Please see below.

Please note that a cosine function $y=A\cos Bx$ has amplitude $|A|$ and period $\frac{2\pi }{B}$. So the function $y=-\frac{1}{2}\cos \frac{\pi }{3} x$ has amplitude $|-\frac{1}{2}|=\frac{1}{2}$ and period $\frac{2\pi }{\frac{\pi }{3} }=6$.