The question says the truck travelled to and fro from the stone quarry.
When going to the stone quarry, the distance was 2.5 km to the east.
On returning, the distance back is also 2.5km
Therefore total distance travelled is 2.5km + 2.5km
= 5km
The displacement 8s dependent on what is the initial and also the final position. The initial and the final position are one and the same thing. Because the truck starts and goes back to the same position, the displacement is zero.
