Chapter 6 - The Circular Functions and Their Graphs - 6.3 Graphs of the Sine and Cosine Functions - 6.3 Exercises - Page 602: 43

$y=-3\ cos(\frac{x}{2})$

As $f(0)\ne0$, use the form $y=a\ cos(bx)$. We can identify the period as $p=2\times2\pi=4\pi$, thus $\frac{2\pi}{b}=4\pi$ and $b=\frac{1}{2}$. We can find the amplitude as $3$, let $a=-3$ which is good for $f(0)=-3$. The answer is $y=-3\ cos(\frac{x}{2})$