First off those are not triangles, tell someone that if you can and it hasn't been said already. Since 6 and 9 correspond we know the big triangle is 1.5 times the size of the small one. Because of this, we can set 1.5(2x-10) or 3x -15 = 15. x = 10. You can also just take 15/1.5 to get 10 but the stuff above is the work.
