Answer: x=13°
Step-by-step explanation: We know total amount of degrees in a triangle is 180°. We also know two of the angles' degrees: 39° and 90°. Now, with this info, we set up an equation: (3x+12)+39+90=180. We add 39 and 90 to get 129; then we subtract 129 on both sides of the equation. Our equation should now look like this: 3x+12=51. Subtract 12 on both sides to get: 3x=39. Lastly, divide 3 on both sides to find the value of x.