Calculate Resonance Wavelength?

Calculate Resonance Wavelength??

Homework Statement

Ive been asked to calculate the resonance wavelength of gas in a resonance tube.
What I have to calculate this with is the frequency f and the average difference of length between each 1⁄2 wavelength measurement. It says calculate it using these 2 things and I cant find a formula to do this with.

The Attempt at a Solution

I know a normal wavelength is v/f but I dont know how to calculate this using the difference between 1/2 wavelength measurements.