Explanation:
o - odd number
e - even number
n + e =o, if n is odd...rule 1
n + e = e, if n is even...rule 2
n × o = o, if n is odd...rule 3
n × o = e, if n is even...rule 4
thus if 3n +6 = o,
then 3n must be odd, following rule 1
=> 3n = o, then n is an odd number, following rule 3