You need to know more...either the angle or the length of one of the sides.
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The one alternative to find the area of a rectangle is when you are given the length of one diagonal and its slope.
The diagonal multiplied by sin(angle) gives one side of the rectangle and the diagonal times cos(theta) gives the other. So the area is (diagonal)2 x cos(theta) x sin(theta).
Divide the rectangle in two triangles and then use the pythagorean theorem to find the remaining sides.
First divide the perimeter by 2 then subtract the diagonal from this. The number left with must equal two numbers that when squared and added together equals the diagonal when squared (Pythagoras' theorem) These numbers will then be the length and height of the rectangle.
If the only known fact is the length of the diagonal then the width and length of the rectangle CANNOT be determined. The diagonal could be that of a square, or of a rectangle that is very long but quite narrow. Consequently at least one more fact is required such as; the dimension of either the length or the width, or the angle that the diagonal makes to the base of the rectangle or even the area of the rectangle.