In the given question it is important to take a look at all the information's that are provided. These information's will help to determine the desired answer. Only thing that needs to be taken care of is the conversion of centimeter to millimeter and then the problem is easy.
Length of lace that Addison needs = 21 cm
Now we already know that
1 cm = 10 mm
Then
The length of lace that Addison should cut = (21 * 10) mm
= 210 mm
So Addison should cut the lace having a length of 210 mm. I hope this is the answer you were looking for. The process should also be easy for you to understand.
