You'd solve this by creating the equation
20 + 6x - 1 + x + 14 = 180
33 + 7x = 180
- 33
7x = 147
÷ 7
x = 21
(because angles in a triangle add up to 180°)
And then you'd substitute in the value of x, so
6 × 21 = 126
126 - 1 = 125° = Angle A
21 + 14 = 35° = Angle C
I hope this helps!