Theory:

The perimeter of a Rectangle: The perimeter is the total distance around the outside, which can be constructed by adding together the length of each side.
apd.png
 
Let us consider a rectangle of length \( l\) units and breadth \(b\) units. Therefore, perimeter of the rectangle \(ABCD\). 
\(Perimeter\) \(P =\) \( (AB + BC + CD + DA)\) units.
\(P =\) \( ( l+ b + l + b )\) units.
\(P =\) \((2l + 2b)\) units.
\(P =\) \( 2 (l + b)\) units.
Thus, the length of perimeter \(l =\) \( P/2 - b\) unit.
And, the breadth of perimeter \(b =\) \(P/2 - l\) unit.
Area of Rectangle: The area of a rectangle is given by multiplying the width times the height.
apd.png
 
Let us consider a rectangle of length \( l\) units and breadth \(b\) units. Therefore, area of the rectangle \(ABCD\).
\(Area\) \(A=\) \( length × breadth\)
\(A =\) \(l × b\) square units. 
Thus, the length of the rectangle \(l =\) \( A/b\) unit.
And, the breadth of the rectangle \(b =\) \(A/l\) unit.
Diagonals of Rectangle: A rectangle has two diagonals they are equal in length and intersect in the middle. The diagonal is the square root of (width squared + height squared).
diagonals.png

\(Diagonal\) \(d =\) \(l2+b2\)
Where \(l\)  is the length of the rectangle.
Where\( b\) is the breadth of the rectangle.