Since all the angles are congruent, the sides must be congruent too.
So, just have one equation on one side equal to another.
5x= 3x+6
Subtract 3x from both sides.
2x=6
Divide 2 into both sides.
x=3.
We can check this by seeing if all the sides are the same.
5(3)= 15
6(3)-5= 15
3(3)+6= 15
I hope this helps!
~cupcake
If you mean parallel to another line, you have to find the the equation of that line. Lets just say it was y= 7x-2. To find the slope of a line parallel to that, then you look at the slope of that line, which in this case would be 7. The slope of a line parallel would have to be the same. So, I can't give you a direct answer because I can't see the graph but that is how you get the answer.