You know the area of a rectangle to be and so

Like they did, let x be the width of the path.

Now consider the length of the fieldandpath - it is going to be the length of the field (70) plus the width of the pathat both ends(2x) which is equal to

Do the same for the width and the path:

The area of the field+path is also length x width:

The area of the path will be the area of the field+path minus the area of the filed: - you are given that and so the equation is

Which can be rearranged into