For the organ pipe
P1 closed at one end the frequency is
v1=(2n+1)‌ where, variables have their usual meanings.
The first overtone frequency will obtain for
n=1 ,
v1=(2(1)+1)‌ v1=‌... For the organ pipe
P2 open at both ends, the frequency is
v2=‌ where, variables have their usual meanings.
The first overtone frequency will obtain for
n=2,
hence, the third overtone frequency will obtain for
n=4,
v2=‌ These frequencies are in a resonance with a given tuning fork. Hence,
v1=v2 ‌=‌ ‌=‌ ‌=‌