Find the depth of the well, in which the sound is heard 0.1 second after the first sound produced.

Let d be the distance of you from water level in well (depth of water level).

The total distance travelled by sound wave =2d

When the wave comes back after reflection so the distance will be doubled

To figure it out total distance, we need velocity and the total time. So please provide it.

If you have velocity it should be Total distance = Velocity • 0.1

I've also calculated your velocity algebrically, (Distance/Time)

Distance/0.1 down

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s