The width of the garden would be found by dividing the area by the length.
Width = 324 / 24 = 13.5 feet.
Now you need to find the total perimeter of the garden:
Perimeter = 2 x length + 2 x width
Perimeter = 2 x 24 + 2 x 13.5
Perimeter = 48 + 27
Perimeter = 75 feet.
He will need 75 feet of fence.
