Question 302471
At first glance, this can seem quite a complex geometrical problem. I always find that drawing a picture very much simplifies the mathematical reasoning. A rectangle has length 120 and width 50. A second rectangle surrounds the first with uniform width. Therefore, the length of the larger rectangle is 120 + 2x and the width is 50 + 2x

The area of the large rectangle = length*width = (120 + 2x)(50 + 2x)
The area of the smaller rectangle = 120*50 = 6000

The difference of these areas will result in the area of the border which runs along the smaller rectangle, or football field.

1800 = (120 + 2x)(50 + 2x) - 6000

You can solve the rest.