posted by Einstein .
One closed pipe organ has a length of 2.5 m.
The frequency of the note is 34.3 Hz. When a second pipe is played at the same time, a 1.7 Hz beat note is heard. By how much is the second pipe too long?
Would you have to use the frequency of the second pipe to find the length of that pipe?
Yes; you need the frequency of the second pipe.
If you are trying to match pipe fundamental frequencies, and the second pipe is too long (rather than too short), then the natural frequency of the second pipe is 34.3 - 1.7 = 32.6 Hz.
Frequency is inversely proporional to length. If the frequency is low by a factor 34.3/32.6 = 1.052, the length is too long by the same factor.