In a triangle of sides a, b and c, the next relation must be satisfied:
The sum of the lengths of any two sides is greater than the length of the third side. That is:
a + b > c
a + c > b
b + c > a
Calling:
a: 2 cm
b: 5.1 cm
c: 2.4 cm
we get:
2 + 2.4 < 5.1
Then, these three segments will NOT form a triangle