Answer:
First, you have to simplify the inequality and solve for x.
x + 2x + 6 < 50
3x + 6 < 50
3x < 44
x < 14 2/3
So, you now have another set of numbers, {13, 14, 15, 16, 17}, and you know x must be smaller than 14 2/3. The only two numbers in that set that works are 13 and 14, making A: {13, 14} the correct answer.