15.3 A steel wire has a length of 12.0 m and a mass of 2.10 kg. What should be the tension in the wire so that the speed of a transverse wave on the wire equals the speed of sound in dry air at 20°C = 343 m/sec.

It is given in the question that:
Length of the steel wire, l=12 m
Mass of the steel wire, m=2.10 kg 
Speed of the transverse wave in the wire=speed of sound in dry airv=343 m/s
Mass per unit length of the wire, μ=ml=2.1012=0.175 kg m-1
For tension T, velocity of the transverse wave:
 T=v2μ= (343)2×0.175 = 20588.575  2.06 × 104 N