As we know that speed of sounds is given as
[tex]v = 332 + 0.6t[/tex]
here we know that
t = 23 degree C
now from above equation we will have
[tex]v = 332 + (0.6)(23)[/tex]
[tex]v = 345.8 m/s[/tex]
now we also know that distance between two consecutive resonance length is half of the wavelength
[tex]L_2 - L_1 = \frac{\lambda}{2}[/tex]
[tex]34 - 12 = \frac{\lambda}{2}[/tex]
[tex]\lambda = 44 cm[/tex]
now we know that
[tex]frequency = \frac{speed}{wavelength}[/tex]
[tex]f = \frac{345.8}{0.44} = 786 Hz[/tex]
so frequency will be 786 Hz