I am lost in Discrete Mathematics. I do not know what is being asked for these two questions.

List five elements in each of the following sets:

2d.) {n Î N | n^{2}+ n is a multiple of 3}

List the (distinct) elements in each of the following sets:

1b.) {x Î Z | xy = 15 for some y Î Z}

I would like verification if this is accurate:

9a.) List all the subsets of the set {a,b,c,d} that contain:

i.) four elements - {a,b,c,d}

ii.) three elements - {a,b,c}, {a,b,d}, {a,c,d}, {b,c,d}

iii.) two elements - {a,b}, {a,c}, {a,d}, {b,c}, {b,d}, {c,d}

iv.) one element - {a}, {b}, {c}, {d}

v.) no elements - {0}