Okay, you now a triangle has 180º degrees and to add to 180º, you have to add all the numbers you have like this:
2x+x+90 (the box is 90º)=180º; add like terms like 2x+x
3x+90=180; cancel 90 by subtracting it and also subtract 90 from 180
3x=90;divide 3 to isolate x and also divide 90/3
x=30 :)