The period is correct, but the amplitude is not. Amplitude must be a positive number. The amplitude of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is given by $\displaystyle{\left|{a}\right|}\usepackage{cmbright}\Huge\$ , and the period equals $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ . The correct answer is: the amplitude is 2 and the period is $\displaystyle{6}\pi\usepackage{cmbright}\Huge\$ .