This graph has the shape of a cosine function that has been reflected around the x-axis. The effect of the negative sign in the equation is to reflect the graph around the x-axis.

The amplitude of this graph and $\displaystyle{y}=-\frac{{1}}{{3}}{\cos{{3}}}{x}\usepackage{cmbright}\Huge\$ is $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ .

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

So the graph and the function match.