The coefficient $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ in front of x will give you $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ of a cycle in the interval $\displaystyle{\left[{0},{2}\pi\right]}\usepackage{cmbright}\Huge\$ . Another way to say this is that the function has a period of $\displaystyle\frac{{{2}\pi}}{{\frac{{1}}{{3}}}}=\frac{{{2}\pi}}{{1}}\cdot\frac{{3}}{{1}}={6}\pi\usepackage{cmbright}\Huge\$ .

It will not affect the amplitude. Therefore, stretching the graph horizontally by a factor of 3 would produce the graph of $\displaystyle{y}={\cos{{\left(\frac{{1}}{{3}}{x}\right)}}}\usepackage{cmbright}\Huge\$ .