You probably made a mistake with the calculation of the period. The amplitude of this graph and $\displaystyle{y}={3}{\cos{{\left(-{2}{x}\right)}}}\usepackage{cmbright}\Huge\$ is 3. However, this graph has a period of $\displaystyle{4}\pi\usepackage{cmbright}\Huge\$ , while $\displaystyle{y}={3}{\cos{{\left(-{2}{x}\right)}}}\usepackage{cmbright}\Huge\$ has a period of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . It also appears that you didn't notice that this is the graph of a sine function. In particular, this graph passes through $\displaystyle{\left({0},{0}\right)}\usepackage{cmbright}\Huge\$ , while the graph of $\displaystyle{y}={3}{\cos{{\left(-{2}{x}\right)}}}\usepackage{cmbright}\Huge\$ does not.