Answer
$$y=3\cos \frac{x}{2}$$
Work Step by Step
Looking at the graph, we find that this is the graph of a cosine function with amplitude $3$ and period $4\pi$. So, we conclude that$$A=3, \quad B=\frac{2\pi }{4\pi }=\frac{1}{2} \\ \Rightarrow \quad y=A \cos Bx = 3 \cos \frac{x}{2}.$$
