Answer:
<em>The length of the longest side is 18 m.</em>
Step-by-step explanation:
Let x = length of the second side.
Then the length of the first side is x - 9.
The length of the third side is 3(x - 9).
Add the three lengths and set equal to the perimeter.
x + x - 9 + 3(x - 9) = 39
2x - 9 + 3x - 27 = 39
5x - 36 = 39
5x = 75
x = 15 (second side)
x - 9 = 15 - 9 = 6 (first side)
3(x - 9) = 3(15 - 9) = 3(6) = 18 (third side)
