Which of the following sets of measures for sides doesn't form a triangle?

14, 18, 18

3, 3, 5

8, 8, 8

7, 9, 17

The sum of the two smallest numbers has to equal a number greater that the largest one. Or, how some people prefer it, the sum of any two numbers of the set has to be a greater number than the third one.

So, the set of sides 7,9,17 does not form a triangle.

because 7+9 = 16, which is less than the greatest number, 17.