The cost to purchase enough of a certain fertilizer to treat a garden is $.25 per square foot of the garden, plus $20.00 for shipping. write an equation describing the cost in dollars to purchase fertilizer for a garden that is 12 feet longer than it is wide. use x to represent the width of the garden.

For this case we have the following variable:
 x: represent the width of the garden
 So, the length of the garden is:
 l = x + 12
 The garden area is:
 A = (x) * (x + 12)
 Then, the cost equation is:
 C = 0.25A + 20
 Rewriting:
 C = 0.25 * (x) * (x + 12) + 20
 
 An equation describing the cost in dollars to purchase fertilizer is:
 C = 0.25 * (x) * (x + 12) + 20