Well, if one side of your first pentagon is 3 meters (assuming that all 5 sides are of equal length), then the perimeter of that pentagon would be 15 m. If your second pentagon is 3 times the size, then the perimeter would be 15(3)=45 m. Knowing your second pentagon also has 5 sides, you would divide the perimeter of 45 by the number of sides (5). 45/5=9 m. The lengths of each side of the second pentagon are 9m