Substitute 21 for x.
y = 21 - 4 = 17
The 1:3 ratio means that the distance from A to the point is 1/4 of the distance from A to B.
The difference of y-coordinates is 10-8 = 2. 1/4 of that is 2·1/4 = 1/2, so the point of interest will have y-coordinate 8 + 1/2 = 8 1/2. This apparently corresponds to the first selection:
... (6 1/2, 8 1/2)
Answer:
2π or 0π are the only points that satisfy the expression, but since the restriction is 0 <= x < 2π
the answer would be 0π
The third one is the right one on e2020