The perimeter is twice the length plus twice the width. Divide the perimeter by two, and you are dealing with length plus width, so simply subtract the width to get the length.
Perimeter is 238 ft. Half of that is 119 ft.
Subtract the width: 119 ft - 54 ft = 65 ft.
P = 2L + 2W = (2 * 65 ft) + (2 * 54 ft) = 130 ft + 108 ft = 238 ft