$\displaystyle{\stackrel{{\leftrightarrow}}{{{G}{I}}}}\usepackage{cmbright}\Huge\$ and $\displaystyle{\stackrel{{\leftrightarrow}}{{{L}{J}}}}\usepackage{cmbright}\Huge\$ are both perpendicular to $\displaystyle{\stackrel{{\leftrightarrow}}{{{H}{K}}}}\usepackage{cmbright}\Huge\$ , so they are parallel to each other.