Not addition, since [tex]-1+1=0[/tex] and 0 doesn't belong to the set.
Not subtraction, since [tex]1-1=0[/tex].
It is closed under multiplication. The set appears to contain all non-zero multiples of 3.
Not division, since dividing some of the numbers results in rationals which do not belong to the set.
What other operations are you supposed to consider?
