The amplitude and the orientation for this function are correct, but the period is not. You may have correctly calculated that the period is $\displaystyle\frac{{{2}\pi}}{{5}}\usepackage{cmbright}\Huge\$ but forgotten how this number relates to the function. Remember that the period of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ .