Draw the angle in standard position: it has its initial side on the x-axis and moves clockwise to the terminal side. The terminal side is in the first quadrant, because the angle measure is between $\displaystyle-{{270}}^{\circ}\usepackage{cmbright}\Huge\$ (the poistive y-axis) and $\displaystyle-{{360}}^{\circ}\usepackage{cmbright}\Huge\$ (the positive x-axis). In particular:

$\displaystyle-{{280}}^{\circ}=-{{360}}^{\circ}+{{80}}^{\circ}\usepackage{cmbright}\Huge\$

This tells you that the terminal side is $\displaystyle{{80}}^{\circ}\usepackage{cmbright}\Huge\$ above the positive x-axis. To say this another way, the x-axis and the terminal side form a $\displaystyle{{80}}^{\circ}\usepackage{cmbright}\Huge\$ angle. This is the reference angle.