The lengths are matched as follows
17, 3, 26: Can be side lengths of a triangle
16, 11, 5: Cannot be side lengths of a triangle
11.1, 20.4, 8.5: Cannot be side lengths of a triangle
21, 7, 6: Can be side lengths of a triangle
How to find the triangle sides
Sets of three numbers, they can represent the side lengths of a triangle if and only if the sum of the two smaller lengths is greater than the longest length. This is known as the triangle Inequality theorem.
We have that:
17, 3, 26: Can be side lengths of a triangle (17 + 3 > 26).
16, 11, 5: Cannot be side lengths of a triangle (5 + 11 is not greater than 16).
11.1, 20.4, 8.5: Cannot be side lengths of a triangle (8.5 + 11.1 is not greater than 20.4).
21, 7, 6: Can be side lengths of a triangle (6 + 7 > 21).
So, two sets can be side lengths of a triangle, and two sets cannot.