Answer
The length of the column is $~~170~m$
Work Step by Step
We can find the time of each pace:
$t = \frac{60~s}{120~paces} = 0.50~s/pace$
Since the soldiers at the back are one pace behind the drummer, it must take the sound a time of $0.50~s$ to reach the back of the column.
We can find the length of the column:
$L = v~t = (343~m/s)(0.50~s) = 170~m$
The length of the column is $~~170~m$
