You have just acquired a $64$ acre farm, in the shape of a square, and divided into an eight by eight array of one acre subplots. You have $21$ crops to plant. Each crop requires its own $3$ acre plot of land, which must consist of three subplots in the shape of a three by one rectangle. Once the crops have been planted, there will be $64-21\times 3=1$ subplot leftover, where you will build your farmhouse.
It isn't the season for planting yet, so you plan to build your farmhouse now, then plant the crops later. Which subplots can you build your farmhouse on which still allow you to plant all of your crops?