Let x be the dimension along the river and y that perpendicular to the river.

Then

x + 2y = 400,   (1)
xy = 1500.      (2)


From (1), x = 400 - 2y. Substitute it into (2). You will get

(400-2y)*y = 1500.

2y^2 -400y + 1500 = 0,

y^2 - 200y + 750 = 0,

 =  =  = .

There are 2 solutions:  1) y = 196.18 ft;  then  x = 400 - 2*196.18 =  15.29 ft.

                        2) y =   3.82 ft;  then  x = 400 - 2*3.82   = 392.36 ft.

Check.  1)  196.18*7.64 = 1499 ft^2  (close !);   2) 3.82*392.36 = 1500 ft^2.

Answer. 1) x = 15.29 ft, y = 196.18;  2) x = 392.36, y = 3.82 ft.