X=b+my solve for m literal equation

Me. i can't figure out this question. give an explanation too. .

Which of the following is not a subset of {1,2,3}?

Three numbers x, y, and z are in the ratio 2: 7: 8. if 12 is subtracted from y, then three numbers form a geometric sequence (in the order x, y–12, z). find x, y, and z. there are 2 sets.