ANSWER
The cost increases $80 per 1 foot increase in side length.
Step-by-step explanation
If 'x' represents the side length of the fence, its coefficient represents how much each foot of that length costs.
Then 500 is the up-front of the project. If they build a fence with a side length of 5 feet or 500 feet, the cost will be at least $500 - then we'll have to add the cost to each foot of fence.