First, 9dm was cut from a skein of rope, then 120V by how many decimeters the length of the rope decreased.

And then – 120 cm.
First, we will translate 9 decimeters into centimeters, remembering that one decimeter “contains” only ten centimeters. Hence:
1) 9 decimetres * 10 cm = 90 cm – cut for the first time.
Next, let’s calculate how many ropes were cut in the first and second times together. To do this, put together the length of the two pieces of rope you cut off.
2) 120 cm + 90 cm = 210 cm.
210 cm = 210 cm / 10 cm = 21 decimetres.
To summarize: the length of the rope has become shorter by 210 centimeters or 21 decimeters.