A farmer plans to fence a rectangular pasture near a river. The pasture must have 180,000 m^2 in order to provide enough grass for the herd. What dimensions would require the least amount of fencing if no fencing is needed along the river.

My Work:

Along the river there is no measurement. Two sides are x and and the third side is (180,000-2x).

Area of rectangle is length times width.

A = x(180,000-2x)

A prime = 180,000-4x.

I then set A prime to 0 to find my critical point.

I found x to be 45,000 but this makes no sense. The correct answer is 600 meters by 300 meters. What am I doing wrong?