The question is invalid; any next two numbers will make a valid sequence.
In fact that is the case with ANY problem like this, unless the statement of the problem tells you what kind of sequence it is.
However, in this problem, a PROBABLE answer can be found by splitting the given sequence up into two sequences:
72, 36, 18, ___
2, 6, 18, ___
Still, even with this apparent pattern, the "correct" answer might be something completely different.