The amplitude of this graph is 2. The amplitude of $\displaystyle{y}={3}{\sin{{2}}}{x}\usepackage{cmbright}\Huge\$ is 3. You also made a mistake with the calculation of the period. This graph has half a cycle in the interval $\displaystyle{\left[{0},{3}\pi\right]}\usepackage{cmbright}\Huge\$ , so it has a period of $\displaystyle{6}\pi\usepackage{cmbright}\Huge\$ . But $\displaystyle{y}={3}{\sin{{2}}}{x}\usepackage{cmbright}\Huge\$ has a period of $\displaystyle\frac{{{2}\pi}}{{2}}=\pi\usepackage{cmbright}\Huge\$ .