Recall the standard form of the equation of electromagnetic wave in terms of the magnetic field, $B_y=B_0\sin \left(kx+\omega t\right) \mathrm{T}$, here $B_0$ is the magnitude of magnetic field, $k=\frac{2\pi }{\lambda } \& \omega =2\pi f$. The equation is given in the question, $B_y=2\times {10}^{-7}\sin \left(\pi \times {10}^{3}x+3\pi \times {10}^{11}t\right) \mathrm{T}$

$\Rightarrow \lambda =\frac{2\pi }{k}=\frac{2\pi }{\pi \times {10}^3}$$\Rightarrow \lambda =2\times {10}^{-3} \mathrm{m}$.