Answer
$y=-3cos(\frac{1}{2} x)$
Work Step by Step
1. Based on the given graph, we can initially identify that it resembles a graph of negative cosine.
2. We can determine the amplitude $|A|=3$, period $p=4\pi$, passing $(0,-3)$.
3. Write a model as $y=-3cos(\omega x)$
4. We have $\frac{2\pi}{\omega}=4\pi$, thus $\omega=\frac{1}{2}$
5. The equation is $y=-3cos(\frac{1}{2} x)$
