the usage of 's and s'

Apostrophe ( ' ) and letter ( s ) is used to make a noun possessive.

For example - That car is amit's
                        The men's hockey match is tomorrow.

• A singular noun can use 's and s' for same effect. 

For example - The car's speed is slow               correct
                        The cars' speed is slow               correct

• A Plural ending with s takes apostrophe( ' ) to become possessive.

For example - The new buses's are fast              correct
                        The new buses's are fast              incorrect