Answer: x=-7 and x=9
Asummung you are solving for x
x+7=0
x=-7
-x+9=0
-x=-9
x=9
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
3+x? Is that your whole question?