Recall the standard form of the equation of electromagnetic wave in terms of the magnetic field, By=B0sin(kx+ωt)T, here B0 is the magnitude of magnetic field, k=\frac{2\pi }{\lambda }&\omega =2\pi f. The equation is given in the question, By=2×10−7sin(π×103x+3π×1011t)T
⇒λ=k2π=π×1032π⇒λ=2×10−3m.