The sides of this triangle are 5x,6x abd 7x,
The perimeter of a triangle is the sum of all sides
We can suggest this inequation:
5x+6x+7x<54
18x<54
x<54/18
x<3
Therefore the lenght of the longest side would be:
7x<(7*3)
7x<21
Answer: the lenght of the longest side would be <21