a) { x ∈ℕ | 15/x is an integer } or { 1, 3, 5, 15 } where ℕ is the set of natural numbers
b) { 0 } or { x | 8x=0 }
c) { x | 2x2=8 } or { -2, 2 } or { x | x2=4 }
I think the green answers best express the statement in words and don't rely on the numeric solutions.