you simplified the expression to give x^a. that's 90% of the problem, right there.
now you're looking for the value of 5^a. that can have lots and lots and lots of different values.
now you're looking for the value of x^0. if x is a positive integer, x^0 is always 1.
if the words "not sufficient" and "sufficient" are indeed reversed in the answer key, please go to this link, select "Errata", and submit your correction:http://www.manhattangmat.com/contact_form.cfm
thanks in advance.