Anyone that knows the answer to this one !

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

Skew lines(segments) are lines(segments) in 3 dimensions that do not intersect the given segment and are not parallel to it.  ae, bf, eg, and fh are your skew segments.  i hope that : )