Chapter 4 - Section 4.5 - Graphs of Sine and Cosine Functions - Exercise Set - Page 596: 64

$$y=-2\cos 2x$$

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$$A=2, \quad B=\frac{2\pi }{\pi }=2 \\ \Rightarrow \quad y=-A \cos Bx = -2 \cos 2x.$$