Answer:
width = 126 ft
Explanation:
I'm going to assume that I am solving for width.
Area = width x length
84 = 2/3 x width
Now divide 84 by 2/3 to get the width (when you divide by a fraction, you actually multiply by its reciprocal, so it is 84 times 3/2)
84 x 3/2 = width
width = 126 feet