Vertical angles are always congruent. Therefore you must set them equal to each other. If we set 2x+20=3x+15 we can do the math like so
2x+20=3x+15
-15 | -15
2x+5 | 3x
-2x | -2x
5 | X
Now to check the answer 2*5+20=30
3*5+20=30
30=30
X=5 is the correct answer. Hope this helped and happy geometry!