Remember that
The Triangle Inequality Theorem states that the sum of any two sides of a triangle must be greater than the measure of the third side
You only need to see that the two smaller sides are greater than the largest side
Verify
1) 7,8 and 9 cm
7+8 > 9 ------> is true
that means
The length sides form a triangle
Classify ------> is a scalene triangle
2) 1.5, 3 and 1.5 cm
1.5+1.5 > 3 -------> is not true
the length sides not form a triangle
3) 4,2 and 8 in
4+2 > 8 ------> is not true
the length sides not form a triangle
4) 5,5 and 5 m
5+5 > 5 -----> is true
that means
The length sides form a triangle
Classify ------>equilateral triangle