Every wave that ever travelled obeys one relation: $v = f\lambda$ — read aloud: v equals f lambda. $\lambda$ is the Greek letter lambda — the Greek L, for the Length of one cycle. The logic is bookkeeping, not physics: $f$ cycles leave each second, each one $\lambda$ metres long, so the front of the train advances $f\lambda$ metres per second.

And the units audit themselves: a hertz is a per-second, so $\mathrm{Hz} \times \mathrm{m}$ is metres per second exactly — the cycles cancel on the way through. Run that check on every rearrangement in this lesson: if the units refuse to cancel into the answer's units, the rearrangement is wrong, no appeal. Units that do work out never prove you right — the check is a one-way street.