Displacement is the distance from the starting position to the final position.
a) In half a year, the Earth travels from one point on the circle to the point on the exact opposite side of the circle (from 0° to 180°). The distance between the points is the diameter of the circle.
x = 2r
x = 2 (1.5×10⁸ m)
x = 3.0×10⁸ m
b) In a full year, the Earth travels one full revolution, so it ends up back where it started. The displacement is therefore 0 m.
