Physics: Vibration Question--completely stumped!?

Question

I have spent about an hour staring at this problem, and while I know the solution is more than likely not as difficult as I'm making it out to be, I just cannot figure this out:

An electric wire that has a mass of 5.0 kg for a 20 m length is strung between two poles 20 m apart. What must be the tension in the wire if it is to take .80 seconds for a pulse to travel along the wire from one pole to the other?

Answer 1

v = 20m/0.80s = 25m/s

v = square root of {(tension) / (mass/length)}

v^2 (mass/length) = tension

(25m/s)^2 (5.0kg/20m) = tension

(625m^2/s^2)(0.25kg/m) = tension

tension = 156.25 kg.m/s^2

tension = 156.25 N