It takes x hours to level the ground and y hours to go uphill, so the distance between A and B is 4x+3y.

Because it is 3 km/h uphill and 6 km/h downhill.

So when the downhill time is 1/2 y,

A-B time x+y B-A time x+ 1/2 y

So x+y+x+ 1/2 y=6.

- 2x+3/2 y=6

- 4x+3y= 12

So AB is 12km apart.