therefore, rtp: both a,b are not divisible by 3 , then ab is not divisible by 3.
then take cases
1: a=3p+1, b=3q+1
2: a=3p+2, b=3q+1
3: a=3p+2, b=3q+2
and prove that ab is not divisible by 3.
then re-state the question and say by contrapositive must be true.
(idk specifically for this question...