Answer:
Pigmented snails: A_B_; albino snails: A_bb, aaB_, aabb
This is an example of recessive epistasis. In recessive epistasis, two recessive alleles mask expression of an allele at a different locus. The 9:7 ratio observed in this question is a special case of epistasis called duplicative recessive epistasis, which means the presence of either aa or bb was sufficient to mask expression of the other gene. In this case, if snails had two copies of either allele a or allele b, pigment production was suppressed, resulting in albino individuals.