d=4w and width = w.
The Pythagorean theorem describes the relationship between the two legs ( a and b) of a right triangle and the hypotenuse (c). In a rectangle, the length and width are the legs, while the diagonal is the hypotenuse.
Pythagorean Theorem:
General Rectangle: 
This specific rectangle with d = 4w: 
Solve for l: 

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