You may have thought that the amplitude was the height of the whole curve, but it is just the height of one hill. Another way to say this is that the amplitude of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is given by $\displaystyle{\left|{a}\right|}\usepackage{cmbright}\Huge\$ . The period equals $\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}\usepackage{cmbright}\Huge\$ . Remember that when you divide by a fraction, that is the same as multiplying by its reciprocal. The correct answer is: the amplitude is 2 and the period is $\displaystyle{6}\pi.\usepackage{cmbright}\Huge\$