Answer:
 ΔABC and  ΔXYZ are SIMILAR by SSS property of similarity.
Step-by-step explanation:
SSS Similarity Theorem:
Two triangles are said to be similar if their  CORRESPONDING SIDES are proportional.
In ΔABC and  ΔXYZ, if  , then  △ABC∼△YZX
  , then  △ABC∼△YZX
Here, in ΔABC and  ΔXYZ
AB = 9, BC = x , AC = 12 
Similarly, XY = 3, YZ = 2, ZX = 4
Here, 

⇒ Corresponding sides are in the ratio of 3, if BC  =6 units
Hence, if BC  = 6 units, then the ΔABC and  ΔXYZ are SIMILAR by SSS property of similarity.