Answer:
7cm, assuming the question is phrased as noted below.
Step-by-step explanation:
I'll interpret "If the length of a rectangle doubles its width." as "If the length of a rectangle is double its width.
Let L be the width, and L the length
L = 2W
The perimeter of a rectangle is 2L + 2W
We can use L instead of 2W, and that gives us:
21 cm = 2L + L
21 cm = 3L
L = 7 cm
