We know that the area is determined by the equation

And the width is

First of all, remember that the area of a rectangle is

Where the length would be

Before we use this length expression, first we need to find the solutions for the given quadratic equation. Using a calculator, the solutions are 2/3 and -2. Only the number 2/3 makes sense to the problem since length can't be negative.
Expressing these solutions as factors would be

Now, we replace all factors to find the length

Therefore, the length is (3x-2) feet.
On the other hand, the perimeter is the sum of all sides, also it can be expressed as

Using the length and width expressions

Therefore, the perimeter is (8x) feet.
When x is 6, the area would be

The area is 128 square feet. Now, if the cost of the fertilizer is $0.25, the total cost for the area would be

Therefore, the total cost is $32 per square foot.