The suffix やすい (yasui) is added to a noun or stem of an i-adjective to create a new adjective that means 'easy to do' or 'inexpensive'. It is commonly used in everyday conversations and written texts.
やすくなる (yasuku naru) - (N4) - やすくなる (yasuku naru) is the potential form of やすい (yasui). It means 'to become easy' or 'to become inexpensive'.
にくい (nikui) - (N4) - にくい (nikui) is the opposite of やすい (yasui). It is added to a noun or stem of an i-adjective to indicate that something is difficult or hard to do.
やすさ (yasusa) - (N4) - やすさ (yasusa) is the noun form of やすい (yasui). It indicates the degree of ease or inexpensiveness.
やすらか (yasuraka) - (N4) - やすらか (yasuraka) is a different adjective that means 'peaceful' or 'tranquil'. It is not related to やすい (yasui) in terms of meaning or usage.