A high-speed photograph of a continuous transverse wave travelling along a tensioned string is shown below.
A student marks the distance x=84 cmx = 84\text{ cm}x=84 cm from the bottom of the first trough to the peak of the fourth crest.
What is the wavelength of the wave?
16.8 cm16.8\text{ cm}16.8 cm
21.0 cm21.0\text{ cm}21.0 cm
24.0 cm24.0\text{ cm}24.0 cm
28.0 cm28.0\text{ cm}28.0 cm