IF I need 50,000 sq feet of fence and the fancy fence(x) along the road is $20 per linear foot and the fence on the other 3 sides is $5 per foot. Find a function for the cost of the fence in terms of x

I'm supposing that the area to be fenced is rectangular/square.

Let x be the side along the road and y be the side next to the side x.

The area is given by xy and 50000 = xy.

The cost of the fence on the x side is 20x.
The cost of the fence on the other sides is 5y + 5y + 5x

The total cost (cost function in terms of x and y) is 20x + 5y + 5y + 5x

Substitute y by x to get the cost function in terms of x only.