Chapter 4 - Section 4.5 - Graphs of Sine and Cosine Functions - Exercise Set - Page 596: 40

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 $y=-3\cos \frac{1}{3} x$ has amplitude $|-3|=3$ and period $\frac{2\pi }{\frac{1}{3} }=6\pi$.