Question

V/2f=l2-l1

Answer 1

For the first resonance, we have

$$l_1=\frac\lambda4.$$

For the second:

$$l_2=\frac{3\lambda}{4}.$$

Thus,

$$\frac\lambda2=l_2-l_1.$$

On the other hand, the wavelength is

$$\lambda=\frac vf,\\\space\\ \frac{v}{2f}=l_2-l_1.$$