Perimeter of the rectangular garden as given in the question = 64 feet
Let us assume the length of the rectangular garden = x feet
Then
The width of the rectangular garden = x - 12 feet
So
Perimeter of the rectangular garden = 2 ( length + width)
64 = 2 (x + x - 12)
64/2 = 2x - 12
32 = 2x - 12
32 + 12 = 2x
2x = 44
x = 44/2
= 22
So
The length of the rectangular garden = 22 feet
And
The width of the rectangular garden = x - 12 feet
= 22 - 12 feet
= 10 feet