The sum of the interior angles of a triangle is always 180 degrees. Thus,
the sum of 31, x+4 and 3x+9 must be 180.
31 + x + 4 + 3x + 9 = 180, or 44 + 4x = 180
While it's not necessary, y ou could reduce this equation before solving for x:
11 + x = 45. Then x = 34 degrees.
The three angles are then:
31
34+4 = 38
3(34) + 9 = 111
Check: show that 31+38+111 = 180 (degrees)