When 15 is divided by y, the remainder is y-3. If y must be an integer, what are ALL the possible values of y?
Try the possibilities:
y can't be 1 or 2 because y-3 would give negative remainders.
if y = 3 the remainder is 0. 3-3=0. So this is one possibility for y.
if y = 4 the remainder is 3. 4-3=1. Nope
if y = 5 the remainder is 0. 5-3=2. Nope
if y = 6 the remainder is 3. 6-3=3. This is a possibility for y
if y = 7 the remainder is 1. 7-3=4. Nope
if y = 8 the remainder is 7. 8-3=5. nope
if y = 9 the remainder is 6. 9-3=6. This is a possibility for y.
I'm not going to list all the rest of them but the other possibility for y is 18.
if y = 18 the remainder is 15. 18-3=15. This is a possibility for y.
so y can equal: 3, 6, 9, or 18.