So originally, let's say the width of Frank's land was [tex]x[/tex] feet.
The area of his land was therefore [tex]x \times x[/tex], or [tex]x^2[/tex], as his land was square.
The width of his land then increased by 3 feet, and the height decreased by 3 feet.
So the new area of Frank's land is:
[tex](x+3)(x-3)\\=x^2+3x-3x-9\\=x^2-9[/tex]
So the new land is now 9 square feet smaller, so it is not a fair deal for Frank.
Hope this helps :)