The idea here is to find out what values for a will make the denominator a 0. And since you can not divide by 0 this will be your non permissible value(s). To find this you take the denominator(after it has been factored down if necessary, this one does not need to be) and set it to 0 and solve for your variable.
9a = 0
a = 0/9
a = 0
Therefore your non permissible value will be,
a ≠ 0