The sum of interior angles of any triangle is 180 degrees. You, probably, know it. Therefore, for your triangle, 54 + 6x + 6x = 180 degs, or 54 + 12x = 180, or 12x = 180 - 54 ---> 12x = 126 ---> x == 10.5 degrees.
If the other 2 angles TOTAL 6x, then x = 21
If the other 2 angles are 6x each, then x = 10.5