Chapter 4 - Section 4.5 - Graphs of Sine and Cosine Functions - Exercise Set - Page 596: 39

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 $y=-4\cos \frac{1}{2} x$ has amplitude $|-4|=4$ and period $\frac{2\pi }{\frac{1}{2} }=4\pi$.