Another Factor Group question

Question: Let G = U17 and H = <[4]> be the cyclice subgroup generated by [4] in G/H. Find the order of Hx where x = 2 or 3.

G = {1,2,3,...,16}

H = {4,8,12,16}

|G/H| = |G|/|H| = 16/4 = 4

o(x)|4 (the order of an element must divide the order of the group). so o(x) = 1,2, or 4

if x = 2 then o(H + [2]) != 1 and 2(H + [2]) = H + [4] = H + [0] so o(H + [2]) = 2

if x = 3 then o(H+ [3]) != 1 or 2 and 3(H + [3]) = H + [12] = H + [0] so o(H + [3]) = 4

Thanks in advance!