Ripples, guitar strings, sound, light: $v = f\lambda$ — read aloud: v equals f lambda — rules them all, which is why it gets called the universal wave equation. The logic fits in one breath: each cycle advances the wave one wavelength, and $f$ cycles happen each second, so the wave covers $f\lambda$ metres of ground per second. That IS its speed.

Run the free check before any arithmetic: hertz times metres is cycles-per-second times metres-per-cycle — the cycles cancel and $\mathrm{m/s}$ falls out, a speed. If your rearrangement's units refuse to cancel like that, it is wrong, no appeal. Units that do work out never prove you right — the check is one-way — but they catch most wrecks before they happen.

One ambush to expect: the paper sometimes hands you a PERIOD where the equation wants a frequency. Flip first — $f = \dfrac{1}{T}$ — then feed the machine.