Your are being asked to find what value of a will result in the denominator being 0, since you can not divide by 0 then this will be your non permissible value. Simply take the denominator(after it has been factored down and reduced, if necessary, it's not in this question though) then set to 0 and solve for the variable.
Therefore your non permissible value will be,
a ≠ 0
Therefore your non permissible value will be,
a ≠ 0