Perimeter is all sides added together. We have the total of 392, so we'll have to work backwards. We know the width of 85 feet, it's rectangle so two of the sides will be 85 feet.
85 + 85 = 170
Subtract 170 from total of 392
392 - 170 = 222
Now just divide to find the length of each length
222 ÷ 2 = 111
111, is the length.
111 + 111 + 85 + 85 = 392
Hope this helps.
