If the length of the field is L meters and the width is W meters then the perimeter (distance around the field) is
2W + 2L
But you know this is 3000 meters so
2L + 2W = 3000 meters.
You are also told that L - W = 50 meters and hence L = W + 50. Hence
2(W + 50) + 2W = 3000
Solve for W.