Answer:
Area is 45000 square feet.
Step-by-step explanation:
Let x be the length of fenced side parallel to the river side.
Let y be the length of other two sides.
Then,

So, we get 
Area of this rectangle = xy
= 

Now attached is the graph of the area function, which is a parabola opening downward.
We can see ta the maximum area occurs when y=
= 150
And 
x =
Therefore, to maximize the area, the side parallel to the river is to be 300 feet long, and the other two fenced sides should be 150 feet long.
The area is =
square feet.