You would need to write a solvable equation, and you can do this because you know that angles in a triangle add up 180° so the missing angle must be (180 - 9x). This makes the equation:
5x + 9 + x + (180 - 9x) = 180
6x + 9 + 180 - 9x = 180
-3x + 189 = 180
- 189
-3x = -9
÷ -3
x = 3
So your answer is x = 3, I hope this helps!
