The coefficient 2 in front of x will give you 2 cycles in the interval $\displaystyle{\left[{0},{2}\pi\right]}\usepackage{cmbright}\Huge\$ . Put another way, this will give you a period of $\displaystyle\frac{{{2}\pi}}{{2}}=\pi\usepackage{cmbright}\Huge\$ . It will not affect the amplitude. Therefore, shrinking the graph horizontally by a factor of 2 would produce the graph of $\displaystyle{y}={\cos{{2}}}{x}\usepackage{cmbright}\Huge\$ .