First you need to find x, and you can do this by creating an equation because you know that angles in a triangle add up to 180°, so:
12x + 12 + 15 + 3x + 18 = 180
15x + 45 = 180
- 45
15x = 135
÷ 15
x = 9
Now you can substitute it in.
Angle A = (12 × 9) + 12 = 108 + 12 = 120°
Angle C = (3 × 9) + 18 = 27 + 18 = 45°
I hope this helps!
