Answer:
Length=16 ft
Width=8 ft
Step-by-step explanation:
Let width be w and length will be 128/w since area is product of length and width
Perimeter will be w+w+128/w. Note that along the river, fencing isn't needed that is why perimeter equals 2w+128/w
Getting the first differential of perimeter with respect to w then

At critical point



Therefore, since length is 128/w, length is 128/8=16 ft
Therefore, maximum length and width are 16 ft and 8ft respectively