The frequency of the first harmonic in the pipe is
[tex]f = 349.2 Hz[/tex]
and the speed of sound in the pipe is
[tex]v=350 m/s[/tex]
So the wavelength of the first harmonic in the pipe is
[tex]\lambda= \frac{v}{f}= \frac{350 m/s}{349.2 Hz}=1.0023 m [/tex]
For a closed-end pipe, the wavelength of the first harmonic is four times the length of the pipe:
[tex]\lambda=4 L[/tex]
Therefore, the length of the pipe is
[tex]L= \frac{\lambda}{4}= \frac{1.0023 m}{4}=0.2506 m [/tex]