This graph has the shape of a cosine function. The amplitude of this graph and $\displaystyle{y}={3}{\cos{{\left(-\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ is 3.

This graph has one cycle on the interval $\displaystyle{\left[-{2}\pi,{2}\pi\right]}\usepackage{cmbright}\Huge\$ , so it has a period of $\displaystyle{4}\pi\usepackage{cmbright}\Huge\$ . The function $\displaystyle{y}={3}{\cos{{\left(-\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ has a period equal to $\displaystyle\frac{{{2}\pi}}{{\left|-\frac{{1}}{{2}}\right|}}=\frac{{{2}\pi}}{{1}}\cdot\frac{{2}}{{1}}={4}\pi\usepackage{cmbright}\Huge\$ .

The effect of the negative sign is to reflect the graph around the y-axis. However, because the graph of cosine is symmetric, this has no effect at all. So the graph and the function match.