The electric field part of an electromagnetic wave in a medium is...

As the coefficient of $x$ is negative, it is moving along +ve $x$-axis and equating the equation \begin{aligned} \mathrm{E}_{\mathrm{y}} & =2.5 \cos \left[\left(2 \pi \times 10^6\right) \mathrm{t}-\left(\pi \times 10^{-2}\right) \mathrm{x}\right] \\ \text{ with } \mathrm{y} & =\mathrm{A} \cos (\omega \mathrm{t}-\mathrm{kx}) \\ \omega & =2 \pi \times 106 \\ \Rightarrow \mathrm{f} & =\frac{\omega}{2 \pi}=10^6 \mathrm{~Hz} \\ \mathrm{k} & =\pi \times 10^{-2} \\ \Rightarrow \quad \lambda & =\frac{2 \pi}{\mathrm{k}} \\ & =\frac{2 \pi}{\pi \times 10^{-2}}=200 \mathrm{~m} \end{aligned}