Essential University Physics: Volume 1 (4th Edition)

We know that $v=\frac{d}{t}$ We plug in the known values to obtain: $v=\frac{1250000}{307.2}=4069\frac{m}{s}$ We also know that $v=\lambda f$ This can be rearranged as: $\lambda=\frac{v}{f}$ We plug in the known values to obtain: $\lambda=\frac{4069}{3.21}=1267.6m$