Since the angles opposite each of the sides given are equal to each other we then can assume that the sides are equal to each other as well.
We can set the sides equal to each other like so...
-6 + 3x = 12
You now must solve for x.
To do this start by adding 6 to both sides
-6 + 6 + 3x = 12 + 6
3x = 18
Now you will divide 3 to both sides
3x/3 = 18/3
x = 6
Hope this helped!
~Just a girl in love with Shawn Mendes