Answer:
The longest sides is 18 m long.
Step-by-step explanation:
Let x meters be the length of the first side.
The length of the second side = x+9 m.
The length of the third = 3x m.
Perimeter = x + 3x + x + 9 = 39
5x + 9 = 39
5x = 30
x = 6 m.
The other 2 sides are 6+9 = 15 m and 3*6 = 18 m.