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