Answer:
Area of the rectangular pen is: 
Step-by-step explanation:
We need to specify that the perimeter of the rectangular area of side x adds to the amount of fence Pablo has (36 feet). Recall as well that the opposite sides of a rectangle are equal, so if there is a side of length x, there must be another one of this size as well (a total of 2 x in the perimeter),let's assume that the perpendicular sides to x are of length y, then:
Perimeter= 2 x + 2 y = 36
so, 2 (x+y) = 36 then (x+y) = 18
and therefore the side y should be:
y = 18 - x
Now we can write the formula for representing the area of the rectangle (product of both quantities: x * y
