That is the length in terms of p and x.
L = p/2 - x
For a rectangle of length L and width W, the perimeter is:
P = 2*(L + W).
Here we know that the perimeter is p, so P = p, and let's say that the width is x.
Then we can write:
p = 2*(L + x)
Solving for L:
L = p/2 - x
That is the length in terms of p and x.
If you want to learn more about rectangles:
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