Hey there!!
Basic formula - SPEED = DISTANCE / TIME
Speed given - 340 m/s
Time = 6.4 seconds
Distance - This is the tricky point : As this is an echo the distance would be doubled. The first distance is when the person shouts and the second distance is when the voice echoes or comes back to the person.
Let's take the distance as ' x '
But with echo, the distance = ' 2x '
Now, getting everything into one equation :
340 = 2x / 6.4
Multiplying by 6.4 on both sides :
2176 = 2x
Dividing by 2 on both sides
2176 / 2 = x
x = 1088
The distance or the depth = 1088 m
Hope my answer helps!!