Okay, so to solve this question, you must know the formula to solve for the area of a rectangle/square.
Area of a rectangle/square - l x w
In this question, you can see the the width (69 m) and the area (5727 m²) is already given. What you can do in this situation is to use the opposite operator (division) to solve for the length of the rectangular field.

You can always double check to see if the number is correct.
A = l x w
= 69 m x 83 m
= 5727 m²