Harmonics are integer multiples of the fundamental frequency.
I found the greatest common factor of 280 Hz and 420 Hz, and that was 140 Hz. Therefore, I knew that this was the fundamental frequency. (280 Hz and 420 Hz are the second and third harmonics, respectively)
The formula for the fundamental frequency of an open pipe is
f= (v/2L)
where v is the velocity of the gas inside the tube and L is the length of the tube.
I assume that I'm ignoring end corrections in this calculation.
f= 140 Hz
v= unknown
L= 1.85 meters
140 = (v)/ (2*1.85)
v= (140)(2)(1.85)= 518 m/s
