Answer:
![105ft](https://img.qammunity.org/2022/formulas/mathematics/high-school/eu9goqmfvcxhra8ipirncxyyiqsx1twy8u.png)
Explanation:
Since it's saying that the width of a rectangle is its length, that means all the sides of the rectangle are equal.
Since a rectangle has only 4 sides, we can divide the perimeter of the rectangle (420 ft) by 4 (sides)
Note: perimeter is the sum of all sides!
420 ÷ 4 = 105
Hope this helps!