velocity is given by:
velocity=wavelength×frequency
We note that the wavelength of a wave is twice the length of the string (2w) in the first harmonic.
velocity=329 m/s
wavelength=2w
frequency=392
substituting the values in our equation we get:
329=2w(392)
329=784w
solving for w we get
w=329/784
w= 0.41964~0.42 m