You probably made a mistake with the calculation of the period. The amplitude of this graph and $\displaystyle{y}={3}{\cos{{\left(-\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ is 3. However, this graph has a period of $\displaystyle\pi\usepackage{cmbright}\Huge\$ , while $\displaystyle{y}={3}{\cos{{\left(-\frac{{1}}{{2}}{x}\right)}}}\usepackage{cmbright}\Huge\$ has a period of $\displaystyle{4}\pi\usepackage{cmbright}\Huge\$ .