The length of David's garden is 48 feet. So, the correct answer is C. 48 feet.
To find the length of David's garden, we can use the formula for the perimeter of a rectangle, which is P = 2(length + width).
Given that the perimeter of the garden is 112 feet and the width is 8 feet, we can plug these values into the formula:
112 = 2(length + 8)
Now, let's solve for the length. First, we need to get rid of the 2 in front of the parentheses. We can do this by dividing both sides of the equation by 2:
112/2 = length + 8
56 = length + 8
Next, we want to isolate the length on one side of the equation. To do this, we subtract 8 from both sides:
56 - 8 = length + 8 - 8
48 = length.