Answer:
 f = 276.6 Hz
Explanation:
This musical instrument can be approximated to a tube system where each tube has one end open and the other closed.
In the closed part there is a node and in the open part a belly or antinode. Therefore the wavelength is
     L = λ/ 4
speed is related to wavelength and frequency
     v = λ f
     λ = v / f
we substitute
     L = v / 4f
     f = v / 4L
the speed of sound at 20ºC is
     v = 343 m / s
let's calculate
     f = [tex]\frac{343 }{4 \ 0.31}[/tex]
     f = 276.6 Hz