To solve this problem we will apply the concepts related to the wavelength of its third harmonic.
It describes that the wavelength is equivalent to
[tex]\lambda = \frac{2}{3}L[/tex]
Here,
[tex]\lambda = Wavelength[/tex]
The wavelength is in turn described as a function that depends on the change of the speed as a function of the frequency, that is to say
[tex]\lambda = \frac{v}{f}[/tex]
In this case the speed is equivalent to the speed of sound and the frequency was previously given, therefore
[tex]\lambda = \frac{343}{262}[/tex]
[tex]\lambda = 1.3091m[/tex]
Finally the length of the pipe would be
[tex]L= \frac{3}{2}(1.3091)[/tex]
[tex]L = 1.963m[/tex]
