Explanation:
length = 2 × width
the perimeter of a rectangle is
2×length + 2×width
and it is 360 yards in our case
2×length + 2×width = 360
and then we use the first equation (2×width = length) in this second equation :
2×length + length = 360
3×length = 360
length = 360/3 = 120 yards