Answer:
x = 7
General Formulas and Concepts:
Pre-Algebra
Order of Operations: BPEMDAS 
- Brackets 
 - Parenthesis 
 - Exponents 
 - Multiplication 
 - Division 
 - Addition 
 - Subtraction 
 
Equality Properties
Geometry
- All angles in a triangle add up to 180°
 
Explanation:
Step 1: Set up Equation
Every angle must add up to 180°.
(6x - 6)° + 54° + 90° = 180°
Step 2: Solve for x
- Combine like terms: 6x + 138 = 180
 - Isolate x term: 6x = 42
 - Isolate x: x = 7