1. The denominator says the the top half of the circle should be divided into $7$ pieces.

2. The numerator says go to the $4$th 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.

$−21\pi$ intersects the unit circle at the point $(−1,0)$, so the signs of the coordinates are $(−,0)$.