Answer:
50°
In order to determine the unknown angle, be sure to use the total sum of 180°
120° - 180°= 60° ( this give you the red side inside the triangle)
Then you simple add the two number inside the triangle
60° + 70° = 130°( then you use this to find the missing corner by subtracting from 180°)
180°- 130° = 50° this is your missing angle X"
To prove it simple add all the 3 angles inside the triangle add up to 180°
60° + 70° + 50° = 180°
X = 50°
