You found the correct amplitude and orientation, but the period is incorrect. Perhaps you thought that the period was $\displaystyle{3}\pi\usepackage{cmbright}\Huge\$ . Remember that the period is the length of one full cycle. The correct answer is $\displaystyle{y}=-{4}{\sin{{\left(\frac{{1}}{{3}}{x}\right)}}}\usepackage{cmbright}\Huge\$ .