I need a little bit more detail in order to answer your question properly
Area of a square with side s is

In your question, the side or s is:

And so the area of a square with that side length would be:

And using this formula:

We get that the area is:

And simplifying that we get the final answer as:
If you look at the data, you can see that every y value is opposite to the corresponding x- value.
So, equation can be written as
y = [-] x or y=[-1]x