your answer has got the term 'a' while the question doesn't .
what i have figured out is that the expression should be
[ ( 1^x + 2^x +................+n^x ) / n ] a/x
i am sure only then the answer (n!)a/n can come
the given limit is of the form 1infinity
so its = elim [ (1^x + 2^x +.......+n^x) /(n) -1 ]a/x ( lim x -->0 everywhere )
= elim[ {(1^x-1) +(2^x-1)........+ (n^x-1)}/x ]a/n
= e[ ln1 + ln2 +............+lnn ]a/n
= e[ ln(n!) ]a/n
= (n!)^(a/n)