The investigators use the tape to mark the borders of the rectangle surrounding the area, so the length of tape is the perimeter of the rectangle.
If the length is x, then the width is ¾x and the perimeter of the rectangle is 2x+3x/2=420, so 7x/2=420, 7x=840, x=120yds and ¾x=90yds, so the area is 120×90=10800 square yards.