This is the graph of a sine function: it has the repeating hill and valley pattern and passes through the origin. The amplitude of this graph and $\displaystyle{y}={3}{\sin{{2}}}{x}\usepackage{cmbright}\Huge\$ is 3.

This graph has 2 cycles in the interval $\displaystyle{\left[{0},{2}\pi\right]}\usepackage{cmbright}\Huge\$ , or a period of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . The same is true for $\displaystyle{y}={3}{\sin{{2}}}{x}\usepackage{cmbright}\Huge\$ , so the graph and the function match.