Afarmer builds a fence to enclose a rectangular pasture. he uses 160 feet of fence. find the total
area of the pasture if it is 50 feet long

Since the problem said he uses 160 feet of fence, you know that is the perimeter
P = 2l + 2w
You know that one of the dimensions is 50, so you can plug that into the equation
160 = 2 x 50 + 2w
160 = 100 + 2w
2w = 60
w = 30
The area is equal to l x w 
50 x 30 = 1500 square feet

The expression of this would be 3.75x+y= 23