The amplitud is the vertical distance from the resting position to the crest or to the trough, and the vertical distance from a crest to a trough is two times the amplitude. Therefore the amplitud (A) is:
A= 4cm/2= 2cm
The period (T) is:
s
The wavelength is defined as the horizontal distance from a crest to the nearest crest or from a trough to the nearest trough. The distance from a crest to the nearest tough was given. Therefore two times that distance is the distance from a crest to the nearest crest.