Answer:
If the length of the rectangle is 2 times the width, we can represent it as L = 2w, where L is the length and w is the width.
Substituting this value into the formula for the perimeter P = 21 + 2w, we have:
P = 21 + 2(2w)
P = 21 + 4w
Therefore, the perimeter P in terms of the width w only is P = 21 + 4w.