An organ pipe is open at both ends. It is producing sound at its sixth harmonic, the frequency of which is 257 Hz. The speed of sound is 343 m/s. What is the length of the pipe

The open pipe has to be of length lambda/2, and odd number.

Fundamental L= fundamentallambda/2
second harmonic L= 3*fundamentallambda/2
third L = 5*fundamentallambda/2

etc.

fundamentallambda= speedsound/freq