Answer:
x = 8
Step-by-step explanation:
The sum of the 3 angles in a triangle = 180°
Sum the 3 angles and equate to 180, that is
3x + 12 + 54 + 90 = 180, simplify
3x + 156 = 180 ( subtract 156 from both sides )
3x = 24 ( divide both sides by 3 )
x = 8