ABC is similar to XYZ. Also, side AB measures 6 cm, side BC measures 18 cm, and side XY measures 12

△ABC  is similar to  △XYZ

So

AB / BC   =   XY / YZ

                                         Plug in  6  for  AB ,  18  for  BC , and  12  for  XY .

6 / 18   =   12 / YZ

                                         Multiply both sides by  YZ .

YZ * 6/18   =   12

                                         Multiply both sides by  18/6 .

YZ   =   12  *  18/6

YZ  =  36

Also notice that to get  △XYZ  ,  take each side of  △ABC  and multiply it by  2 .