Tension in an oscillating string

[momopeaches]

Homework Statement

A 120-cm-long, 3.0 g string oscillates in its n = 4 mode with a frequency of 150 Hz and a maximum amplitude of 5.5 mm. Wavelength is 0.6 m.

What is the tension in the string?

Homework Equations

f = sqrt(TL/m)/2L

The Attempt at a Solution

150 = sqrt(1.2T/.003)/2.4

Solving for T gives me
T = 324
Which, according to the program(Mastering Physics), is wrong. I'm not sure if this is the right equation, but its the only found I could find that uses all of the information I have, excluding amplitude.

 

[Gear300]

The 2L only works if the string is at the fundamental harmonic. More generally, the velocity of a wave is v = f*l (in which f is frequency and l is wavelength) and v = Sqr(T/(m/L)), in which T is tension, m is mass, and L is string length.

 

[momopeaches]

Ah, thank you! I got it now. I was assuming 343 m/s for the velocity. Should I not assume, unless the problem explicitly says "through air"?