Half the perimete

Let the length and breadth of garden be 'x' and 'y' respectively. 
Given, 
Half the perimeter is 36 m
We know perimeter of Rectangle = 2(length + breadth)
Half the perimeter = x + y
⇒ x + y = 36
⇒ x = 36 - y     [1]

Also, Length is 4 m more than width
⇒ x = y + 4     [2]
From [1] and [2], we have
36 - y = y + 4
⇒ 2y = 32 
⇒ y = 16 

Putting in [1], we have
⇒ x = 36 - 16 = 20

Hence, Length and Breadth of garden are 20 m and 16 m respectively.