Answer:
15.8 cm 
Step-by-step explanation:
In 20.8 seconds, the number of vibrations = 42 
In 1 second, the number of vibrations = 42 / 20.8 = 2.019 
Frequency is defined as the number of vibrations in one second.
f = 2.019 Hz
Crest travel 371 cm in 11.6 s
v = 3.71 / 11.6 = 0.319 m / s
Use 
v = f x λ
λ = v / f = 0.319 / 2.019 = 0.158 m = 15.8 cm