Data Sufficiency

Hey guys,

I have been trying to decipher Prob 169 from the 2nd ED quant guide and its been driving me nuts.

Question:
where N is a positive integer;
N^2 is 72;
Identify the largest positive integer that must divide N;


I understand the explanation (more comsi, than comsa), but I am unsure if I can make all these assumptions on the test.

Any explanations from fellow Gmatters is much appreciated.

Thanks
KK