The wavelength is given by:
[tex]\lambda=\frac{v}{f}[/tex]where v is the velocity and f is the frequency. In this case the velocity is that of the light (since we have electromagnetic radiation), and the frequency is 2.5e7, then we have:
[tex]\begin{gathered} \lambda=\frac{3\times10^8}{2.5\times10^7} \\ \lambda=12\text{ } \end{gathered}[/tex]