Answer:
Part A : D.) 
Part B : Length of the sandbox is 10 feet.
Step-by-step explanation:
Given,
Perimeter = 29 ft
We need to find the equation for the perimeter and also the length of the sandbox.
Solution,
Let the width of the sandbox be 'w'.
Now as per question said;
The length of the sandbox is 1 foot longer than twice the width of the sandbox.
So we can say that;
Length = 
Now we know that the perimeter is equal to the sum of twice of length and width.
framing in equation form, we get;
Perimeter = 
we have given the perimeter, so on substituting the value, we get;

Hence The equation used to find the width is
.
Now we solve for 'w'.
Applying distributive property, we get;

Subtracting both side by '2' we get

Dividing both side by 6 we get;

Width of the sandbox = 4.5 ft
Length of the sandbox = 
Hence Length of the sandbox is 10 feet.