I used the method described in books from

MGMAT (prime box)

for N to be divisible by 9, it must contain in its prime box two of 3's

1) 18 is a factor of N^2

prime factorization of 18 = 2 * 3^2

n^2 = n * n

which means that one n, in its prime box, will take a two, and a three, since we do not know whether n might contain another 3, statement 1 is insufficient.

2) same method used, we have that n has only one 3, it might have more than one, but we do not know, hence the statement is insufficient.

1+2 -> we know for sure that n has one three in its prime box, but it might have more than one, therefore 1+2 insufficient.