A harmonic wave is traveling along a rope.

The oscillator that generates the wave completes 50.0 vibrations in 26.6 s. A given crest
of the wave travels 355 cm along the rope in a
time period of 12.1 s.
What is the wavelength?
Answer in units of m

wave equation. Memorize this.

frequency*wavelength=speed
50/26.6 * wavelength=3.55/12.1

solve for wavelength in meters.