Angles in a triangle always add up to 180, so you could write an equation to solve for x:
31 + x + 4 + 3x + 9 = 180
44 + 4x = 180
- 44
4x = 136
÷ 4
x = 34
So now that you have the value of x, you can use substitution to find angles S and T.
34 + 4 = 38° = angle S
34 × 3 = 102
102 + 9 = 111° = angle T
I hope this helps!