Question

{0, 2.6, 4.2, 5, 10, 15}

Answer 1

{10, 15} and {5, 10, 15}.

Trying out some values:
x=10, 5*10 + 3 = 53, so 10 and above satisfy the inequality.

x=5, 5*5 + 3 = 28, which the lowest value that will satisfy the inequality so nothing below 5 will.

So any set with 5 or higher are correct, so
{10, 15} and {5, 10, 15}.