This is easier to understand with a picture...
We know that a triangle has 3 sides right?
Well, if the perimeter is 39 and there are 3 sides you would divide 39 by the 3 sides
39/3=13
Therefore, one side of the triangle would equal 13
We know that a triangle has 3 sides right?
Well, if the perimeter is 39 and there are 3 sides you would divide 39 by the 3 sides
39/3=13
Therefore, one side of the triangle would equal 13