1. The denominator says the the top half of the circle should be divided into $7$ pieces.
2. The numerator says go to the 4th piece starting from ($1, 0$)
3. This angle is the 2nd quadrant so the signs are ($−, +$) for ($x, y$).

Move clockwise from ($1, 0$).

Divide half of the circle into $9$ pieces. Every $18$ pieces is one time around the circle.

$-21π$ intersects the unit circle at the point ($-1, 0$), so the signs of the coordinates are ($-, 0$).