SaySis a set of elements (finitely or infinetly). Suppose thatSis closed under the operation*,and that the associative law holds. Ifa*e=aand there exists-a(the inverse element) such thata*(-a)=efor allainS.Thene*a=aand(-a)*a=eand the uniqueness ofe(the identity element) can be deducted.

A little help with this is needed. Thanks in advance.