The amplitude of $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ is given by $\displaystyle{\left|{a}\right|}\usepackage{cmbright}\Huge\$ , so the amplitude of $\displaystyle{y}=-{2}{\cos{{\left(\frac{{1}}{{3}}{x}\right)}}}\usepackage{cmbright}\Huge\$ is $\displaystyle{\left|-{2}\right|}={2}\usepackage{cmbright}\Huge\$ . The period equals

$\displaystyle\frac{{{2}\pi}}{{\left|{b}\right|}}=\frac{{{2}\pi}}{{\left|\frac{{1}}{{3}}\right|}}=\frac{{{2}\pi}}{{1}}\cdot\frac{{3}}{{1}}={6}\pi.\usepackage{cmbright}\Huge\$