You do this by creating an equation with the expressions you've been given, and then solving:
3x - 14 + x + 90 = 180
4x - 14 + 90 = 180
- 90
4x - 14 = 90
+ 14
4x = 104
÷ 4
x = 26
Then you substitute 26 into (3x - 14):
3(× 26) - 14 = 78 - 14 = 64
So your answer is 64°
