$\begin{gathered} v=343\text{ m/s}=343000\text{ mm/s} \\ f=2.6million\text{ Hz=2600000 Hz} \\ v=\lambda f \\ \text{Solving }\lambda \\ \lambda=(v)/(f)=\frac{343000\text{ mm/s}}{\text{2600000 Hz}}=0.1319\operatorname{mm} \\ \text{Then wavelength is }0.1319\operatorname{mm} \end{gathered}$