You have to use scale factor. 6 is 2/3 of 9. Using this, we can say that x is 2/3 of 15. So, x must be 10 (because 10 is 2/3 of 15). We can also say that y is 2/3 of 18. So, y has to be 12 (because 12 is 2/3 of 18).
You are making all sides of the triangle smaller by a scale factor of 2/3, because 9*2/3=6. For x, we can do 2/3*15, which equals 10. For y, we can do 2/3*18, equalling 12.
