In cycle group if a is generator then it will generate all elements. i.e. a^{10}= e

If a generator then (a^{8})^{x} = (a^{10}) = e

put x= 1,2,3,4,5,6,7,8,.... take smallest x which satisfy this

(a^{8})^{5} = (a^{10}) = e

(a^{10})^{4} = (a^{10}) = e

8 also satisfy but order will be smallest such value satisfy the conditionl. hence 5 is answer