4L = 64
Explanation: The perimeter of a parallelogram (like most other quadrilaterals) is defined as;
P = 2 (L + W)
Where P is the perimeter, L is the length and W is the width. Note that the width is given as 4 metres less than its length, hence if the length is L then the width shall be L-4. The perimeter can now be expressed as follows:
P = 2 (L + [L-4])
If the perimeter is 72, then
72 = 2 (L + L - 4)
72 = 2 (2L - 4)
By expanding the bracket on the right hand side we now have;
72 = 4L - 8
Add 8 to both sides of the equation
72 + 8 = 4L - 8 + 8
80 = 4L
Therefore the equation is 4L = 80
And the Length is 20 metres.