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The distance between the 1st crest and 2nd crest is called wavelength.  Hence 2 cm is the wavelength.

i suppose there is some thing missing in the question.  Perhaps  the 15 crests and 15 troughs are produced in 1 second.

Hence frequency then in that case is -- 15 per second or 15 Hertz.

the speed of the wave is   v = frequency * wavelength
                 = 2 cm * 15 Hz = 30 cm/sec

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