Answer: 18 meters
Let L be the length, w the width (all in meters).
Then you have L = 2w - 4, and 2l+2w =100. or L + w = 50. But L from the first relation into the second and simplify: 3w = 54 or w = 18.
Therefore, the width is 18 meters.
Happy to help; have a great day! :)