It's a fact that the sum of the angles of a triangle add up to 180º.
So, 55º + 85º + (3x+4)º = 180º.
That's the equation you need to solve:
55 + 85 + 3x+4 = 180 (combine like terms)
144 + 3x = 180 (subtract 144 from both sides)
3x = 36 (divide both sides by 3)
x = 12 (you've got x)