The given graph (Fig. 2) shows the displacement versus time

Here $v=1500 \mathrm{~m} \mathrm{~s}^{-1}$

$T=2 \mu s=2 \times 10^{-6} \mathrm{~s}$

$\therefore$ Frequency,

$v=\frac{1}{T}=\frac{1}{2 \times 10^{-6}}=5 \times 10^{5} \mathrm{~Hz}$

Using $v=v \hat{\lambda}$, we get

$\lambda=\frac{v}{v}=\frac{1500}{5 \times 10^{5}}=3 \times 10^{-3} \mathrm{~m}=3 \mathrm{~mm}$