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 field and path - it is going to be the length of the field (70) plus the width of the path at 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