word problem 3 (rectangular)

a rectangular field is to be fenced on all 4 sides and then divided into two fields by placing a fence parallel to two opposite sides. if the total amount of fencing is 200 meters, what dimensions will give the maximum enclosed area?

i thought since it has been divided by 2 fields, now the # of slides are 8.

so i divided 200 by 8 to get a length of the side, which is 25... so thought the answer would be 25 x 25 x 25.

but then i realized it is rectangle...

how do i do this question?!