Final answer:
The correct formula to find the perimeter of a rectangle is P = 2l + 2w. If we rewrite this in terms of length (l), it would be l = (P - 2w) / 2. The provided options do not match the correct formula.
Step-by-step explanation:
The formula for the perimeter of a rectangle is usually written as P = 2l + 2w, but in the question, it seems to be incorrectly stated as P = 21 + 2w. Assuming it's a typo and meant to be 2l, to rewrite the formula for the perimeter of a rectangle in terms of l (length), you would isolate l on one side of the equation. The correct formula would be P = 2l + 2w. If we isolate l, the equation becomes l = (P - 2w) / 2. So the correct equation rewritten in terms of l is l = (P - 2w) / 2, which unfortunately is not one of the provided options.
Regarding the given examples, they seem unrelated to the initial task of rewriting the formula in terms of length l.