You have seen that the function equals 0 at $\displaystyle\theta={0},\pi,{2}\pi,{3}\pi,\usepackage{cmbright}\Huge\$ and $\displaystyle{4}\pi\usepackage{cmbright}\Huge\$ . The graph of sine has a hill/valley pattern that repeats forever in both directions. Each hill and each valley is $\displaystyle\pi\usepackage{cmbright}\Huge\$ units wide and ends on the horizontal axis, or with a value of 0. So the function will continue to be equal to 0 at every multiple of $\displaystyle\pi\usepackage{cmbright}\Huge\$ . All the multiples of $\displaystyle\pi\usepackage{cmbright}\Huge\$ in the interval $\displaystyle-{3}\pi\le\theta\le{3}\pi\usepackage{cmbright}\Huge\$ are $\displaystyle\theta=-{3}\pi,-{2}\pi,-\pi,{0},\pi,{2}\pi,{3}\pi\usepackage{cmbright}\Huge\$ .