Change to degrees:
We subtract 360° enough times to get a
a positive coterminal angle less than 360°:
585°-360° = 225° (it was only necessary to
subtract 360° one time.)
That is in QIII, So to get the reference
angle, we subtract 180° from 225° and get
225°-180° = 45°.
The tangent is + in QIII, so
Edwin