## Precalculus (6th Edition) Blitzer

$$y=-2\cos 2x$$
Looking at the graph, we find that this is the reflected graph of a cosine function about the $x$-axis with amplitude $2$ and period $\pi$. So, we conclude that$$A=2, \quad B=\frac{2\pi }{\pi }=2 \\ \Rightarrow \quad y=-A \cos Bx = -2 \cos 2x.$$